Less true than you’d think. America outspends the rest of the world by an incredible margin and could probably happily halve its budget without being invaded by Canada or Mexico. Many European countries have spectacularly (too) low levels of defence spending and are still doing Ok, because they’re free-riding on NATO. Ukraine spent quite a lot, and spent it well, but Russia’s so much larger it doesn’t make enough of a difference.
But yeah, there are many things we could better be spending the defence budget on than galaxy formation research.
You almost had it. The US spend a good portion of the defense budget propping up NATO which is why European countries can get by with spending less. Love him or hate him, what Trump was trying to do in getting NATO to pull its fair share in defense spending was a good idea. Sure the US could spend waaay less and still ensure it won't get invaded (they'd have to come by sea or from Canada or Mexico). But Europe would piecemeal become part of a new Russian or Chinese Union.
You almost had it. Obama during his two terms pushed other NATO members to increase their spending to the 2% GDP target. They agreed and set long term goals to increase their spending and started to do so.
Trump stupidly described other NATO members as "owing" the US. Not only had several countries met their spending goals by the end of the Obama admin most of the alliance was on track with increased spending including targeted equipment upgrade spending. Trump's complaints were that other NATO members hadn't met their goals ahead of the agreed upon schedule. This was going to be used for a pretext for withdrawing the US from NATO.
NATO funding is not a simple issue. There's direct and indirect expenditures that "fund" NATO. It's not some protection racket. Funding can be direct funding I.e. military units/equipment maintained as a rapid response force or indirect funding I.e. spending to upgrade or buy equipment to keep up with the overall norms of the alliance.
But yeah, there are many things we could better be spending the defence budget on than galaxy formation research.