The Union Army, also known as the Grand Army of the Republic, the Federal Army, or the Northern Army, was the military of the Union (the United States) during the American Civil War, during which time the US Army was split between loyal Unionists and the secessionist Confederate States Army. The Union Army consisted of the permanent regular army of the United States, while augmented by temporary units of volunteers and conscripts. Over the course of the war, 2,128,948 men enlisted in the Union Army, of whom 8.4% were African-Americans; 25% of the white men who served were immigrants and another 25% were first-generation Americans. 596,670 Union soldiers were killed, wounded, or missing during the war.
At the start of the American Civil War, the US Army consisted of ten infantry regiments, four artillery regiments, two cavalry regiments, two dragoon regiments, and one mounted rifle regiment, scattered widely across the United States. 179 of the US Army's 197 companies occupied 79 isolated posts in the American West, while the remaining 18 manned garrisons east of the Mississippi River, most of them on the Canadian border or on the East Coast. Only 16,367 servicemen (including 1,108 officers) were serving at the time, and 20% of them resigned and joined the Confederate States Army. At the time, most regular soldiers came from the lower tiers of society, including recently-arrived Irish immigrants such as Daniel Hough, the first casualty of the war. After the secession of the American South, President Abraham Lincoln called for loyal states to raise 75,000 troops for a three-month campaign against the Confederacy and to defend Washington DC. Following the First Battle of Bull Run, Congress decided to support a volunteer army of up to 500,000 troops, and the call for volunteers was easily met by patriotic Northerners (including both Republicans and War Democrats), abolitionists, and immigrants who enlisted for a steady income and meals. 100,000 Southern Unionists served in the Union Army during the war.
The South's secession and the firing on Fort Sumter motivated Americans from across the political spectrum to take up arms. Even in New York City, where 65.17% of the electorate had voted for the Democratic/Constitutional Unionist ticket (and a mere 34.83% for the Republican ticket), the public immediately rallied to the Union cause, and even Democratic leaders such as James Gordon Bennett and (begrudgingly) Fernando Wood took up the Union cause. Thousands of Democrats, who subsequently refused to lay aside their party, united with the Republicans at Union rallies, and Republicans, Douglas Democrats, Breckinridge Democrats, Mozart Hall men, and Bell-Everttites all joined. However, the Union defeat at the May 1863 Battle of Chancellorsville and the draft quotas imposed on New York aroused both Copperheads and War Democrats, and Democratic dissent once again began to grow.
By the 1864 presidential election, many soldiers had grown weary of their political leaders, with many of them considering abstaining. Most soldiers intended to vote for the candidate who would end the war quickly and honorably; some soldiers expressed distaste at voting for George B. McClellan due to his association with Copperheadism, and many Democrats intended to vote for Lincoln out of a desire to bring down secession, but other soldiers were willing to vote for a candidate who could make an "honorable peace" with the Confederacy. While most soldiers were ready to vote for Lincoln's re-election in March 1864, many were discouraged by the bloodshed of the Overland Campaign and were ready to vote for anyone who would put an end to the butchery. The fortunes of war dictated most soldiers' political feelings, with the Union victories of late 1864 helping persuade many soldiers to decide to vote for Lincoln. MacClellan was widely popular among the soldiers until William Tecumseh Sherman's capture of Atlanta. Soldiers who voted for McClellan saw themselves as Unionists while rejecting the abolitionism of the Republican Party, with many soldiers expressing disgust at being slaughtered to free African-Americans.
The Army of the Potomac was rife with Democratic sympathizers, including at the leadership level, resulting in the Republicans engaging in voter intimidation by court-martialing Democratic soldiers for distributing Democratic campaign literature in camp, while distributing only Republican newspapers. In fact, many Democrats who voted for Lincoln in 1864 saw their Republican vote as a one-time act, seeing Lincoln's administration as representing the friends of the country, and the Democrats its enemies. Most Democrats in the army opposed both the Copperheads of their party and the abolitionists of the other. Somewhere between 50 and 60% of the eligible soldier voters voted for Lincoln, substantially lower than the often cited 78%, with many Democrats having a real indifference or aversion to participating in elections. Hundreds of soldiers were kept from voting by skirmishes or even by proscription from their officers; at times, only Republican tickets were distributed to the soldiers. The Republican-controlled army dismissed several pro-McClellan officers and punished openly Democratic soldiers, even though many protested at being labelled Copperheads.
Lincoln's assassination in 1865 transformed him into a sort of national deity, and many Democratic veterans later expressed remorse for not voting for him. Union veterans overwhelmingly voted Republican after the war, with their affiliation being based on postwar issues and not on the war policies of the 1864 GOP.