Where does defense stop in an integrated world? Aircraft carriers and a strong navy are necessary to protect waterways and freedom of navigation, which the peaceful side of USA absolutely needs in its pursuit of freedom, peace and liberty. East Asia's history is a great example of what happens when you focus only on your internal world and close off to the rest of the planet. Also why they're not falling for the same mistake again and not having these qualms in the modern era.

Peace requires policing, a credible deterrent to abuse. On the world's stage, there's no actual hegemon for the planet. You gotta choose who will police by mere "I'm stronger than you". I think the US is the most acceptable police, at least it's an elected democracy with individual freedom at its core.

At the end of the day, a government is whoever is strong enough to prevent the other one from governing: provide protection and services to your people in exchange for being the boss. Roaming bands overthrow each other, still visible in Afghanistan and Syria, and dictate who's effectively Government on a given day in a given area. Who applies the laws, who provides justice, who keeps you safe. Scale that up to modern nations and we look at it as if it was the natural order of things: just stick to you backyard and all will be good. But no, it was never like that and even less so these days: the world is shrinking rapidly.

Can't have peace without deterrence against bad actors. And deterrence doesn't stop only on your turf.