A huge part of our spending. It has been considered necessary and sacrosanct.
But that depends on our objectives.
Certainly, protecting the citizens of the US and the US property and country is the purpose of defense.
Beyond directly defending ourselves, there is some question.
We do not necessarily have the right to interfere in the sovereign affairs of other nations, except to the degree it is granted by the United Nations that a higher value is to take precedence: that of many lives being at stake.
We do not need to be the "policeman of the world". We are interfering busybodies, potentially, and are considered to be arrogant in that we try to impose our values upon others.
I would recommend that we cut defense spending in 1/2, at least 10% a year until accomplished, and that a conference of business people and top military people act as a panel to judge the efficiency of what is done.
A priority list should be made up, relative to what is most effective for the dollar spent.
WHAT DOES NOT WORK
It has never worked for us to go into another country to try to tame it, as those who live there are much smarter in maneuvering and can fight, fight, and fight - until we are exhausted. Vietnam, Iraq, Afghanistan - what more proof do we need.
On the ground, hand to hand contact is something where we have an advantage but not the greatest advantage.
Loss of lives
And it is unacceptable to incur the loss of priceless lives for some intangible value. The only time when it is justified to risk our lives would be in fighting against something that would cause us to lose more lives.