The United States has been the target of terrorists for some time. There are fascists and communists in the world still- as well as genocidal racists, imperialists and criminals of sundry sorts. Those all require defense strategies. I will mention a couple of points, though not reply to all.
Just in May there were about 133,000 people arrested trying to cross the U.S. border illegally. Maybe an equal number made it across. Some of those are terrorists; many are criminals. U.S. security faces challenges from nuclear weapons including suitcase devices as well as biological attacks and more. Russia only really closed down the Biopreparat facilities of the former Soviet Union completely about 1999. Some of those programs migrated elsewhere.
Russia has some fine weapons systems that are have global attack coverage. So does China and others…ICBMs. The anti-nuclear weapons proliferation sentiment that developed after World War Two when the U.S.A. was the sole nuclear power and could have taken the opportunity to annihilate Russia, China and others if it wanted to has not been shared by several countries. Even so the Middle East and the world will not be more secure if Iran develops nuclear weapons too.
Nazi and Japan did declare war on the United States to commence the Second World War. Chinese mass casualties were nothing to brag about; the point is to win a war with as few casualties as possible- the enemy can have the casualties.
The C.I.A. developed from the Office of Strategic Services (O.S.S.) following the Second World War. It’s primary task was to counter communist expansion around the globe. The Cold War was a global conflict itself with wars against communist insurgencies and infrastructure permeating underdeveloped countries. The Cold War required perpetual vigilance against the Soviet bloc-Warsaw pact nations. That was very costly. Africa became saturated with weapons. Soviets tried to install nuclear missiles in Cuba. Che Guevara and other Cubans fomented revolutions in Latin America and Africa. The Khmer Rouge perpetrated genocide, etc. Unfortunately post-Cold War socialists still exist to subvert democracy through other means. They present an opportunity cost to the free world that is substantial; the need to reform Democracy and capitalism to make it more egalitarian, ecologically sustainable and more effective is obscured and neglected with the continuing stupid neo-Marxist paradigm of dialectical clash between capitalism and socialism continuing.
There are far too many poorly educated people that work intentionally or unintentionally for a neo-authoritarian socialist global political system. And then there is the Muslim problem of theocracy, religious law and world conquest philosophy.