I am kind of trying to watch 2 things regarding US allies. Short of engaging a real trade war, they could stop buying US weapons like the F-35, though honoring existing contracts. Countries could also start withdrawing their modest number of soldiers from Afghanistan as a form of protest.
It is very difficult to take actions like this. For one thing, there are existing contracts in place that project years into the future. Mechanics and pilots have to be trained. Replacements have to make sense from a military perspective.
What Trump is trying to do here is shake our allies down in the same way that shady real estate developers engage in under-the-table deals. It involves a lot of push-and-pull theatrics. You make a lot of empty threats and pretend to be intransigent while looking for some kind of advantage. He has no idea of the complexities involved and probably doesn't care. This is the style of behavior he is comfortable with. When things get seriously broken, we probably won't know it until it is too late to repair the damage easily. Trump is not only weakening America. He is weakening the entire Western alliance.