The Marines are like my West Highland Terrier. They get up every morning, they want to dig a hole, and they want to kill something.
An economically confident America has - since becoming a world power at the start of the 20th century - tended toward global engagement. It is during times of economic stress (1930s, 1970s) that America has become more withdrawn.
Transnational terrorism, in the form of the Salafi Jihadist movement, is fundamentally a function of globalization.
Despite living in this post-9/11 age of transnational terrorism, the risk of death during air travel has plummeted to the point where we now measure it in the 'per billions' of passengers.
Some Western demographers have posited, due to the female shortage created by the one-child policy, that China will be forced to field a vast force - as in tens of millions strong - of wifeless men who'll gladly wage wars around the planet to burn off all those unrequited hormones.
During the cold war, it was easy for the Pentagon to justify its budget, as the Soviets essentially sized our forces for us. We simply counted up their stuff and either bought more of the same or upgraded our technology.