The Civil War starts and James Garfield decides he should be a colonel (soldiers got to elect their own colonels in Ohio). What were his qualifications? A few years in the state senate and a massive ego. He was not elected to colonel because he had shown too much interest in it, which was considered ungentlemanly, and he pouted about that for a while. Then he got diarrhea.
James Garfield always had diarrhea. It was his main way of dealing with stress and emotion. His biographer notes at one point that you could tell the story of his life by how his bowels reacted to it. This is delightful to me because he had such a high opinion of himself, but he was just diarrhea all the time. He had to take breaks from being in the Civil War because of his diarrhea. He had surgery at one point during his Congressional career — all I know is that he was confined to bed for six weeks after an operation for a "painful rectal condition," but I don't know if that was the cause or the effect of the diarrhea. Either way, it was serious.
When he was done pouting and started being in the Civil War for real, he was fairly successful, but never as much as he wanted to be. He was desperate to win glory for himself. He hated it when there were long stretches between battles because how would America learn how great he was if he couldn't distinguish himself in battle? He started being like, I hate war! I want to be in the US Congress instead! But he couldn't make his mind up to leave the army because surely that pivotal battle that would make him a hero was just around the corner.
Eventually he met General William Rosecrans, who liked him immediately and offered him a job as his chief of staff. He hemmed and hawed about it because it would be more glorious to lead his own troops into battle, but eventually took the position because of the political prestige (and because having a more sedentary job would be more suited to his chronic diarrhea). It didn't quite work out that way, because Rosecrans soon became another one of the generals that Lincoln and Secretary of War Stanton could not stand. Even Garfield eventually became annoyed at Rosecrans' lack of initiative, but had to officially remain loyal to him because of his position (although that didn't stop him from writing a letter to Treasury Secretary Salmon P Chase about how annoyed he was with Rosecrans, which blew up in his face later). After a particularly disastrous display of lack of leadership at the Battle of Chickamauga, Grant was promoted above and allowed to fire Rosecrans, and Garfield basically left the army after that to run for Congress.
He was elected to Congress in 1862 and stayed there until 1879, when he was elected to the Senate for only a year before running for president. The war chapters were the last that Margaret Leech wrote before she died, and the Congress and President chapters were finished by someone else, more summarily than she would have. As a result, I know a lot less about his years in Congress than I do about his early years and war years. This may be to Garfield's disadvantage, because he was a very popular and influential member of Congress, and he and Crete were very happy once she moved to Washington with him. It could be that attaining this stature mellowed him out a bit, or that moving to Washington gave Crete a little more personality. His manic need for approval may have been salved by being a Congressman.
Even so, he was almost no one's first choice for the Republican nomination in 1880. The Republican were split between a New York-led faction that wanted Grant for a third term, and a more moderate faction that wanted James Blaine of Maine. Garfield was at the convention to support his Ohio friend John Sherman (who was Hayes' Secretary of State), but the delegates' votes were deadlocked, with neither Grant's or Blaine's supporters willing to switch over, so Garfield was proposed as a compromise candidate and quickly got the nomination. They all seem to be compromise candidates these days.
He was inaugurated in March of 1881, shot in July, and died in September. In his three "active" months in office, the only thing he accomplished was standing up to Roscoe Conkling, but it will be more interesting to talk about that in the context of Chester Arthur, who was caught in the middle of that fight, and became president upon Garfield's death.