British military might rested on its navy for centuries, Paul Kennedy reminds us in The Rise and Fall of British Naval Mastery, which I am just finishing. That changed almost exactly 100 years ago with the advent of the submarine. "There is no doubt that this new weapon almost brought the British Empire to its knees," he writes.
By 1937, British spending on the RAF passed spending on its army, and a year later, also passed the navy's budget.
Thomas E. Ricks covered the U.S. military for the Washington Post from 2000 through 2008.