Based on date of death, I assume Isaac was wounded during the Atlanta campaign or during operations in Northern Georgia or Alabama. Chattanooga was a major support depot for Sherman's army until he left Atlanta and began his march to Savannah.

The history of the 125th Ill. has been reprinted but I don't know how available it is. Your library might be able to get a copy through inter-library loan.

Atlanta, Ga. Campaign May 1-Sept 8. Tunnel Hill May 6-7. Demonstration on Rocky Faced Ridge May 8-11. Buzzard's Roost Gap May 8-9. Battle of Resaca May 14-15. Rome May 17-18. Operations on line of Pumpkin Vine Creek and battles about Dallas, New Hope Church and Allatoona Hills May 25-June 5. Operations about Marietta and against Kenesaw Mountain June 10-July 2. Pine Hill June 11-14. Lost Mountain lune 15-17. Assault on Kenesaw June 27. Ruff's Station, Smyrna Camp Ground, July 4. Chattahoochie River July 5-17. Peach Tree Creek July 19-20. Siege of Atlanta July 22-Aug 25. Utoy Creek Aug 5-7. Flank movement on Jonesboro Aug 25-30. Battle of Jonesboro Aug 31-Sept 1. Lovejoy Station Sept 2-6. Operations against Hood and Forest in North Georgia and North Alabama Sept 29-Nov 3.

125th Illinois, "The History of the One Hundred Twentyfifth Illinois Volunteer Infantry," by Robert M. Rogers, Second Sergeant, Company B. Originally released in 1882 at Champaign, Ill, this first edition copy costs $ 125. 226 page reprint with portaits now available for $35.

I did find reports in the Official Records of the War of the Rebellion

MAY 1-SEPT. 8, 1864.--The Atlanta (Georgia) Campaign
Report of Capt. George W. Cook, 125th Illinois Infantry

HEADQUARTERS 125TH ILLINOIS VOLUNTEERS
Atlanta, Ga., Sept. 7, 1864
SIR: In compliance with orders, I have the honor to submit the following report of the part taken by this command in the recent campaign, from its commencement, in May, until its arrival at Atlanta, on the 4th instant:

In order for me to make this report nearly accurate I must depend mainly upon the notes of Col. O. F. Harmon and Lieut. Col. James W. Langley, respectively, commanders of the regiment from the commencement of the campaign until the 1st day of the present month, when the command fell to me. Forming a part of the 3rd Brigade, 2nd Division, XIV Corps, this regiment, commanded by Col. O. F. Harmon, numbering 449 effective men, left Lee and Gordon's Mills, Ga., on the 3d day of May last, where it had been doing, in connection with the brigade commanded by Col. Daniel McCook, outpost duty, and marched to Ringgold, where it joined the division then commanded by BrigGen. Jefferson C. Davis. After a day or two of rest at the last-named place the forward march was resumed and continued in until we faced the enemy before Buzzard Roost. On the road to this point we skirmished with the retreating foe at different points. At Buzzard Roost we were most of the time during several days actively engaged with the enemy's skirmishers, but lost no men. The regiment participated in the flank movement through Snake Creek Gap, which move gained Buzzard Roost. This march was long and tedious, but was borne by all cheerfully and without complaint. At Resaca we met the enemy and were engaged actively with him. The regiment occupied a temporary line of works immediately in front of and but a few hundred yards from the works of the opposing forces. Here, as in previous instances, every man did his duty, until the flight of the enemy from Resaca, on the night of the 14th [15th] of May opened on the following morning a new field of labor. An expedition to Rome, Ga., was fitted out for our division, and on the morning of the 15th [16th] the regiment was detailed, with one section of Battery I, Second Illinois Artillery, to command and guard the division supply and ordnance train, in rear of the marching column of the division, to that city. The regiment took no part in the fight at Rome on the 17th; arrived with its important charge on the following day. Remained at Rome doing various duty until the 24th day of May, when the entire division took up its line of march toward Dallas; joined the Army of the Tennessee, to which the -division was temporarily attached, near the last-named place on the 26th day of May. The next encounter with the enemy was at Dallas, on the night of the 27th of May, when we were attacked by a superior force while engaged in relieving the 22nd Indiana, who were doing picket duty. The enemy succeeded in capturing, owing to the unavoidable unadjusted condition of the lines at the moment, 14 enlisted men and 1 commissioned officer, and wounding 3 others (enlisted men); but this temporary disaster was quickly, though but partially, compensated by the capture of 1 captain, 1 lieutenant, and 25 enlisted men from the enemy. On the following morning the pickets drove the enemy back with a loss of 20 killed and wounded; but 2 men wounded on our side. Until we reached Kenesaw Mountain, on the 27th of June, nothing worthy of note occurred, although we daily faced the foe. At Kenesaw Mountain, on the morning of the 27th of June, the regiment, in connection with the brigade, formed a part of the attacking column that was on that day hurled against the enemy's works. The 125th Regiment was the foremost in the brigade. The conflict was short and bloody, and it is painful to record that a repulse to our forces along the entire line was the result. Never fought troops better than on that day, and attention is called to the casualties in this command alone, which were 120 in the short space of 20 minutes, nearly one-half of which were in the list of killed; and also that the brigade rallied within 60 yards of the enemy's works, threw up intrenchments under a heavy fire, and held them until the night of July 3, when the enemy evacuated their lines and retreated toward Atlanta. In the above-mentioned engagement we lost several brave officers and men, chief among whom was the colonel of the regiment, of whom it may not be inappropriate here to remark that a braver soldier or more efficient officer in line of his peculiar duties the Army of the Union does not contain. The command at this point devolved upon Lieut. Col. James W. Langley, who commanded the regiment through the engagement at the Chattahoochee River, July 5, and at Peach Tree Creek, on 19 July, in both of which it was actively engaged, and subsequently until, in the midst of the battle of Jonesborough, September 1, when Colonel Dilworth, brigade commander, was wounded and carried off the field, the command of the brigade devolved upon him (Lieutenant-Colonel Langley), and that of the regiment upon myself. During the whole of the engagement at Jonesborough the officers and men behaved with that high courage which marks the soldier fighting in support of a good cause. The regiment, while yet under command of Lieut. Col. James W. Langley, reached the crest of the hill just in front of a rebel battery, engaged the enemy in the open field, and contributed greatly in aiding the Second Brigade, on our left, to scale the enemy's works. Here they fought with the desperation of men determined to win, and they did win, though not until my regiment had lost one officer and 3 men killed, 30 enlisted men wounded--some 6 or 8 of them mortally. At dark my regiment was formed in line with the 85th, 86th and the 110th Illinois, when we built a strong line of works. The troops were marched to Jonesborough and put in temporary camp. The regiment and brigade was ordered to Atlanta, Sept. 4, in charge of nearly 2000 prisoners, captured in the Jonesborough fight, and went into camp at this place, where it now remains, and it is sincerely hoped that, if the campaign is over, it will remain until, in the opinion of the powers that be, it is needed in the field for active operations. I respectfully submit and herewith transmit a list of casualties in the command since May 3 up to the close of this campaign.
In conclusion I would say in behalf of the officers and enlisted men of this regiment, that they, with few exceptions, most manfully and soldierly, in every engagement in which the regiment has participated, stood up and faced the foe, while many fell dead on the field. It would be difficult to make special mention of names and do ample justice to all and injustice to none. A grateful country will reward them all for their noble services.
The survivors of Kenesaw and subsequent battles can never forget our patriotic dead. Colonel Harmon, Captains Fellows and Lee, and Lieutenant McLean fell at the former place, where duty called them. At Peach Tree Creek, Lieutenant Jones, of Company D, commanding Company B, died as he had lived--a true Christian soldier. Lieutenant White, who so nobly fell at the crossing of the Sandtown road, was loved and respected by all whose good fortune it was to have his acquaintance. Again, at Jonesborough, the daring and faithful Captain Charles fell in the discharge of his immediate duty. So with Sergeant Thralls, who for more than two months had commanded Company B: wounded in the leg during the hottest of the engagement, received his fatal blow from a stray bullet while his wound was being dressed. My confidence in him as a company commander was always firm, because I knew him to be a brave man.
Very respectfully, your obedient servant,
GEO. W. COOK, Captain, Commanding Regiment.
Capt. CHARLES SWIFT, Actg. Asst. Adjt. Gen., 3d Brig., 2d Div., 14th Army Corps.
-----
List of casualties in the 125th Rgt. Illinois Volunteer Infantry from May 8 to Sept. 6, 1864
O Officers. M Men.

-Killed.- Wounded. -Missing.-
Date. Place of engagement. O M O M O M
May 11 Buzzard Roost, Ga. .... .... .... 1 .... ....
May 15 Resaca, Ga. .... .... .... 1 .... ....
May 31 Dallas, Ga. .... .... .... 5 1 14
June 27 Kenesaw Mountain, Ga. 4 50 .... 63 1 6
July 5 Chattahoochee River .... .... 1 .... .... ....
July 19 Peach Tree Creek, Ga. 1 1 .... 3 .... 1
August 5 to 12 Before Atlanta, Ga. 1 .... 3 13 .... ....
September 1 Jonesborough, Ga. 1 6 .... 29 .... ....

Total 7 57 4 115 2 21

Respectfully submitted,
GEO. W. COOK, Captain, Commanding
Sept. 29-Nov. 13, 1864.--Operations in North Georgia and North Alabama.
Report of Lieut. Col. James W. Langley, 125th Illinois Infantry, commanding 3rd Brigade

HDQRS. THIRD BRIG., SECOND DIV., 14TH ARMY CORPS,
Near Savannah, Ga., January 3, 1865.
CAPTAIN: Pursuant to orders of date December 28, 1864, I have the honor to report substantially the operations of my command from the fall of Atlanta up to and including the fall of Savannah.
On the 4th day of September last my command, comprising the Eighty-fifth, Eighty-sixth, and One hundred and twenty-fifth Illinois, the Fifty-second Ohio, the Twenty-second Indiana, and three companies of the One hundred and tenth Illinois Volunteer Infantry, moved from Jonesborough, in charge of the Fourteenth Army Corps hospital trains and about 1,600 prisoners of war, to Atlanta, reaching the latter place about sundown. After reporting to Major-General Slocum, then commanding at Atlanta, and turning over my rebel prisoners, by his order, I went into camp to the left of the White Hall road, just beyond the suburbs of the city, where I remained until the 29th day of September, when I received orders to move to Chattanooga by ears as rapidly as possible. At about 9 p.m. I got the brigade on board a train of cars going north, and arrived at Chattanooga October 1 at 3 p.m. General Morgan having previously gone to Huntsville, Ala., with the First and Second Brigades of the division, I reported in person to Major-General Thomas, who directed me to proceed to Huntsville as rapidly as I could do so with safety. I accordingly started on a train from Chattanooga at sundown and arrived safely at Huntsville at 12 m. October 2, and went into camp on the south side of town, where we remained about two hours, when I received orders to move to the depot immediately. On going to my command I found it all asleep, a luxury the men had not enjoyed since leaving Atlanta three days before. Shortly after arriving at' the depot we commenced reloading the same cars from which we had disembarked but two or three hours previously. At dark the entire division moved toward Athens six miles, where we met with obstructions on the road that delayed us until next morning, when we again started, reaching within six miles of Athens by noon. Here were other obstructions in the road, and we disembarked for the last time. By the order of General Morgan I directed Captain Cook, commanding the One hundred and twenty-fifth Illinois, to guard the empty trains back to Huntsville and return to the brigade by the next train. At 2 p.m. the brigade marched with the division to Athens, and went into camp in two lines, facing southeast. It will be seen that my command was three days and four nights traveling from Atlanta to Athens, and with the exception of about six hours of that time they were on the cars so closely crowded that none could lie down. Even had that been possible they could not have slept, owing to the very heavy rain that fell almost without intermission during the trip. It was the ill-fortune of my brigade to ride on trains managed by drunken, incompetent, and irresponsible conductors and engineers, who for the most part seemed perfectly indifferent to the suffering and inconvenience they imposed upon the soldiers, so long as they enjoyed official confidence, a profitable salary, and plenty to eat.
On the morning of the 4th of October the brigade, except the One hundred and twenty-fifth Illinois, not yet returned from Huntsville, and the One hundred and tenth Illinois, in charge of supplies, marched from Athens toward Florence, fording Elk River at Brown's Ferry about sundown, after which it marched four miles to Rogersville and encamped for the night. October 5, marched at 6 a.m., crossed Shoal Creek at dark and went into camp beyond and within seven miles of Florence. October 6, the First Brigade moved toward Florence at 7 a.m., but met the enemy's cavalry skirmishers just beyond our picket-line. I was immediately ordered to follow with three regiments, leaving one as camp guard. It was with little difficulty that a reconnaissance was pushed through to Florence, as it was afterward ascertained that the enemy opposing our progress thither amounted to only about 150 men. We reached the town at 1 p.m. and theft learned certainly, what we already began to suspect, that Forrest had escaped across the Tennessee River. At 3 p.m. we returned to Shoal Creek, reaching there about sundown. On the following morning the entire command, with the division, returned to Florence and went into camp on the southeast side of town, where we remained three days. Up to this time, from the date of our departure from Atlanta, it rained heavily every day, rendering the roads from Athens to Florence very muddy, besides swelling the numerous streams to their banks. These streams we were compelled to ford, with the exception of Shoal Creek, which had a good bridge. The men were drenching wet, adding greatly to the weight of their loads, and their sleep, though sound, was the sleep of exhaustion and afforded them but little rest; besides, many were barefoot and footsore. Those who fell sick by the wayside were left in houses to the care of the citizens, as we had no means of transportation. The citizens, as far as I was able to learn, uniformly treated our sick soldiers with a great deal of kindness. Officers had no comforts or convenience above those of enlisted men, and indulged only in such luxuries as they could transport by virtue of their own physical strength. In short, I do not hesitate to say that our trip from Atlanta to Florence came nearer tasking to the utmost the physical endurance of the American soldier than any campaign I ever witnessed, and I cannot withhold the meed of praise so eminently due the brave officers and men who made it. They performed all that was required of them with that characteristic cheerfulness which has recently become so strongly engrafted in the heart of-our army. October 10, we left Florence on our way back toward Athens; marched nineteen miles and encamped for the night. October 11, resumed march toward Athens, and encamped for the night within eight miles of the town. October 12, reached Athens at 10 a.m., where the One hundred and twenty-fifth Illinois rejoined the brigade. We moved two miles east of town, and went into temporary camp. At 2 p.m. I received orders from General Morgan to detail one regiment to rebuild a railroad bridge across a branch of Limestone Creek, which next morning it had completed, and about noon cars began to arrive at Athens to transport the division to Chattanooga. My command got off at 4 a.m. October 13, and reached Chattanooga at midnight, where I encamped the brigade in two lines on the west side of town, where it remained four days. My first effort was to procure shoes and clothing for my barefooted and almost naked men; I succeeded in procuring 785 pairs of shoes and a sufficiency of clothing. At the expiration of our four days' rest at Chattanooga we marched on the morning of the 18th of October, by the Rossville and Dry Valley roads, to Lee and Gordon's Mills, on the Chickamauga River, and encamped for the night. October 19, marched to La Fayette and encamped for the night. October 20, marched south from La Fayette about twelve miles and turned to the right on a road leading to Broomtown Valley, and, after fording Chattooga River, we went into camp for the night. Oct. 21, marched 25 miles on Broomtown Valley road toward Gaylesville. Oct. 22, marched to Gaylesville and went into camp north of town, where we joined the corps and remained five days, subsisting partly on Government rations and partly from the country. Oct. 28, left camp at 2 p.m. and marched eight miles toward Rome, and camped for the night at Missionary Station, near the State line. Oct. 29, resumed the march at 6 a.m. and encamped at night on the north side of Oostenaula River, and within one mile of Rome, where we remained until Oct. 31, when I received orders to march to Kingston as guard to corps train. My command left Rome at 3 p.m.; marched eight miles, and encamped at night on Cedar Creek. Resumed march at 7 o'clock next morning, and reached Kingston within three hours, where I went into camp and remained until Nov. 8, on which day we marched to Cartersville and encamped southwest of town, about two miles, until Nov. 12, when I was ordered to relieve a brigade of the 2nd Division, XV Corps, at the fort on the north side of the Etowah River. Nov. 13, marched to Acworth, stopping on the way to tear up and destroy three miles and a half of railroad track and ties; reached camp at 9 p.m.

I have the honor to be, very respectfully, your obedient servant,
JAMES W. LANGLEY, Lieutenant-Colonel 125th Illinois Vol. Infty., Comdg. Brigade.
Capt. T. WISEMAN, Assistant Adjutant-General.