This is so irrelevant to the conversation, that it indicates you don't understand what's at issue or the basic geopoltical terms in which to evaluate US strategic capabilities.

The US isnt trying to prevent invasion, it's trying to dominate every region of the world. It's military is extremely over-sized to merely defend america. It's extremely undersized to dominate every region of the world in 2025. This is why comparing sizes of militaries is irrelevant and extremely misleading. Essentially all other militaries are concerned with only local defence and power projection.

From ~90s to 00s the US military was big enough to dominate the world, because it had no rivals. When you have rivals even half your size, to dominate them, you need massively out-class them. Consider that during the cold war the US spent 10% of its GDP, vastly more in real terms than the soviet union.

China can dominate its region of the world very cheaply compare to the US dominating *china* ! because defence is vastly cheaper than offence and geographically local power projection is relatively cheap. China is not designing a military to contain all of south america -- the US *is*

The US is trying to maintain arms to entirely ensure its own defence under any possible threat *AND* dominate russia in eastern europe, china in the south china sea, the middle east, ensure all shipping lanes are open, staff miltiary bases throughout europe, asia, etc. -- and the vast array of proxy countries in which it maintains a military pretence. There are 100k troops in europe, 40k+ in japan, and so on.

The US doesn't evaluate its military capability in terms of "what happens if mexico invades"