Since defense has the largest piece of the pie, it is most likely to get cut when other entitlement programs start running out of money.
I think that depends on who is holding (the most) power when that decision comes to the forefront. I doubt that forcing large numbers of highly visible professionals (read: servicemembers) into a poor economy will attract political support from the military itself or from pro-military demographics. There's several historical examples of similar problems occurring (the demobilization of the Continental Army in 1781 and the Bonus Army in 1932). A strong and publicly popular Chief of Staff could (attempt to) preserve the institution against an unpopular and divided Congress. Truman, for example, I believe holds the lowest ever recorded popularity for a President after his dismissal of MacArthur in 1951. The general pacifism of the 1930s does not exist in strength, and so I think it would be extremely difficult to make drastic and lasting cuts to the defense budget without first changing America's attitude toward the military and its on-going operations around the world.