American Revolution (1776-83) cost $2.4 billion
War of 1812 (1812-15) cost $1.55 billion.
Mexican War (1846-49) cost $2.38 billion
Civil War (1861-65) cost $79.74 billion (75 percent for the Union forces)
Spanish American War (1898-99) cost $9.04 billion.
World War I (1917-21) cost $334 billion
World War II (1941-45 ) cost $4.1 trillion
Korean War (1950-53) cost $341 billion
Vietnam (1965-75) cost $738 billion
Persian Gulf War (1990-91) cost $102 billion
Afghanistan War and Other War-On-Terror Operations (2001-10) cost $321 billion
Iraq War (2003-10) cost $784 billion
These are official numbers from the Pentagon.
One, despite the complaints that the Iraq War is the cause of our budget problems, note that the total costs of waging the war (although this number for Iraq and other wars don't count the cost of caring for veterans, of course) match what we spent at the stroke of a pen for the stimulus package enacted in early 2009.
Second, as Strategypage addresses, when you consider our GDP, the burden of the spending continues to go down. The absolute number is interesting but not really relevant if the issue is whether it is a burden on our economy. And that doesn't even begin to factor in the financial cost of losing a war or letting enemies win by not even fighting.
Third, always remember that we spend money to avoid spending blood--ours and the enemy's. Let me know if you want to send our troops in without body armor or MRAPs or top-of-the-line M-1s. Let me know if you think it is acceptable for wounded troops to wait hours before being evacuated to get medical help. Let me know if you think it costs too much to train them. Heck, let me know if you want to get rid of expensive precision weapons and rely on dumb bombs and shells that get more of our people killed running many more shells and bombs to the shooters, kill more of our people with friendly fire and more effective enemy fire because we move slower and the enemy isn't dead or suppressed, and even kills more of the enemy and innocent civilians.
On the bright side for the cost issue, of course, fighting more cheaply would mean that far more of our troops would die in war rather than survive and add to the post-war costs of treating veterans. If you are focused on dollar cost alone, of course.
We could have a much cheaper military. Are you really willing to pay for it?