> Bulgaria, Estonia, Finland, Latvia, Lithuania, Poland and Romania are in NATO. "Defensive alliance" means the US bankrolls and guarantees their security, there is no two way street with them, they can do nothing for us.
They can do a lot of harm when used against you. The part of Europe, which had fallen under Russian domination at the end of WWII, is where the most advanced Soviet weapons were developed and manufactured, because it had the most educated population and most the skilled specialists. Romania, for instance, produced a significant amount of electronics for Soviet weapons. After the USSR collapsed, these factories were converted to producing control boards for Western vacuum cleaners and microwaves. Would you rather see Russia overrun Romania and convert these factories back to producing military radars, missile guidance systems, and sonars for nuclear submarines whose sole purpose is to sneak up on American cities and set them on fire? The Soviet S-75 SAMs on Cuba that shot down the American U-2 spy plane piloted by Rudolf Anderson had Romanian electronics inside, as had many other weapons that killed Americans all over the world. Ukraine manufactured the best Soviet nuclear missiles, now the very same factories are subcontractors for NASA. Poland was known for its shipyards, Bulgaria was a powerhouse of ammo manufacturing.
Denying Russia access to this pool of talent for the promise of helping those countries in case of a Russian invasion is an incredible bargain, especially considering that such a guarantee sharply reduces the risk of invasion in the first place. You get it all for free, as long as the guarantee remains credible enough to deter the invasion. All the US president has to do is appear believable when he says that he'll nuke Moscow if Russia invades Poland. That doesn't cost much.