It may have been briefly slashed after the lost Vietnam war. Are you saying the current spending (more than half the total global war spending) is a recent development? And that even assuming just 20%, can the US economy truly carry that?
You are right, war spending is 'only' about 20% of the total budget. Not including downstream effects of wars, so it would be interesting to know the real percentages. Wouldn't be surprised if the effective percentage would be 30 or higher.
I don't see the relevance of defense spending as a percent of "total global war spending" since it says nothing of the USG's ability to afford such costs -- and I understand your primary assertion to be about affordability. Defense spending as a % of GDP is a much more sensible variable to evaluate.
Defense spending as a % of GDP has remained below 1970 levels ever since the post-Vietnam cuts, and currently stands at about 60% of that value -- a whopping 4% of GDP. Further, it is slated to decrease at least 10% once the draw down from the Bush Administration's wars is complete (defense spending has always declined significantly after a period of conflict is over).
Therefore, it just isn't the case that the US suddenly piled on a bunch of unsustainable defense commitments after the Vietnam War, nor that what it has spent on defense since then is the primary determinant of future budgetary issues: spending on welfare programs has averaged 7% of GDP over the same period and is slated to increase
substantially over the next 20+ years.
P.S. I'm sure we both agree that defense spending is too high. But that isn't what is being debated here.
edit: checked the data and tweaked a few values (1970 instead of 1975, 4% instead of 4.4%, 60% instead of 63%)