Different reasons for different times for different peoples. No different for any other nation (country). So examples follow:
1. America warred with Britain because it wanted to be its own country (Revolutionary War 1776).
2. The US warred with Britain again because the UK disrespected America by kidnapping US sailors on the high seas.
3. The US warred with the American Indian because the US wanted his land.
4. The US warred with Mexico to because the US wanted his land too; under the slogan of "Manifest Destiny."
5. The US warred with its southern brother in the Civil War because the Rebel wanted to start his own country...and the northern brother wouldn't have it! So they fought about it. And the Northern brother won in 1865.
6. The US warred with Spain because the US wanted Cuban leasing rights, Puerto Rico, the Philippines, etc. and become one of the "big boys on the block" (a fellow colonizer like the Europeans), so when the battlewagon USS Maine exploded in Havanna Harbor 1898...it was war (Spain was blamed for the Maine's sinking...later proved to be an accidental coal explosion).
7. The US warred with Germany in 1917 because Germany stated they were going back to "unrestricted sub warfare" & and the US warned them about that...so war it was.
Just a few examples.
The American civil war.
More Americans were lost than in any other war. And Americans were fighting Americans.
The Americans lost the Vietnam War (1954-1975).
The revolutionary war.
In the Revolutionary War, The Americans fought against the British.
Which was an important part of Americans war efforts on the home war?Read more: Which_was_an_important_part_of_Americans_war_efforts_on_the_home_war
The most Americans died in Vietnam which was also the longest war that America was involved in.
The cold war was not a war; it was a military stand-off (no war). Americans died to operational causes (accidents).
sorry, but i do know that 50,000 Americans were killed in the Korean War.
What the newly freed African Americans needed to do what after the war
WHY DID THE AMERICANS ATTAK
5 Americans