Truman re-joined the Missouri National Guard, organized an artillery battery, and was soon elected 1st Lieutenant. When his unit was absorbed into the United States Army, Truman had to take his first regular Army physical. Truman passed the eye examination by memorizing the chart; he was basically blind in this left eye, with uncorrected vision of 20/400.
On 13 April 1918, the George Washington arrived at Brest, France. While the Great War was raging at the front, Truman basically enjoyed himself, surrounded by luxuries that were new to him. Truman was assigned to an artillery school near General John Pershing's HQ in Lorraine (a region in France that borders Belgium, Luxembourg, and Germany), where Truman experienced even more luxury. Soon enough, military reality set in with Truman going through rigorous college-level classwork, going from class-to-class for over 12 hours a day. Truman's classroom/field training centered on the French 75mm field gun; although the gun recoiled, the aim kept true. The French 75mm could fire 30/40 shots a minute at a range of 5 miles; the fire from a battery of four was absolutely murderous. No other field piece in World War I could match the French 75mm; to the Germans, it was the "Devil Gun".
American Expeditionary Force was filled with men that, to put it mildly, resisted authority.
Truman was terrified when he first presented himself in front of his men, but he didn't tolerate any discipline problems, and Truman made it clear right away that he was in charge, saying that they had to get used to him, not the other way around. While Truman made it clear that he wouldn't put up with any shenanigans, he improved the food and took the time to get to know his men, which helped his command immensely.
Truman didn't budge, and he screamed at his mean using profanity, something they hadn't heard from him. A livid and terrified Captain Truman succeeded in getting his men and the situation under control. Battery D had lost four horses and the 75mm guns were in mud up to their axles and were impossible to move with manpower alone. Truman decided that the guns could be saved later, and he took his men to a far safer location, where they slept.
Ardennes. On 16 September 1918, Truman was ordered to take his men to the Ardennes to be part of the 24 mile long line of US soldiers whose goal was to deny the Germans access to the railhead at Sedan; it would be known as the Meuse-Argonne Offensive, which was by far the largest American military attack in US History up to that point (The overall strategy for Meuse-Argonne was organized by an officer on Pershing's staff, George C. Marshall, who would become an indispensable member of President Truman's Cabinet).
Once in position, Truman spent the next two-plus days getting ready for "H-Hour", which occurred at 5:30 am on 26 September 1918, with each battery firing 1000 rounds an hour (or about 6 rounds per gun per minute), and each gun would need to be idle 10 minutes every hour in order for the barrel to cool. After the initial salvos, the idea was to advance and keep firing while being sure to not hit any of their own men in the process, as if moving the guns forward on that terrain under fire wasn't already hellishly difficult enough.
Afterwards, Truman went back to "Super Happy Fun Time" in Nice, and then in Paris, in effect on leave. When Truman returned to his command, the wait to head home became endless, but it was during that time that General Pershing arrived and shook Truman's hand, telling him that he did a good job, and to get his men safely out.
Truman was already thinking about running for political office in Missouri by using his outstanding war record, but mostly he just wanted to get back home and to marry the love of his life, Bess Wallace. But Truman, like most of the other American soldiers, were stuck in France while President Woodrow Wilson was in negotiations at Versailles. Finally, on 9 April 1919, Truman sailed for New York City with over 1000 men of the 129th Field Artillery on the former German passenger ship, Zeppelin.