This is something many liberals don’t want to touch, simply on the premise that because Trump picked it up, it must be toxic. Trump actually does have a point here, but in typical fashion he really hasn’t thought it through very much.
First, there really is something here. Other NATO members do indeed spend less on the military (both per capita and as a fraction of their GDP) than does the USA. Since money spent on the military has to come from somewhere, that quite naturally means that the money thus spent cannot be spent on other things. As Eisenhower once said:
Every gun that is made, every warship launched, every rocket fired signifies, in the final sense, a theft from those who hunger and are not fed, those who are cold and are not clothed. This world in arms is not spending money alone. It is spending the sweat of its laborers, the genius of its scientists, the hopes of its children.
So yes, common Americans would be better off if their government spent less on the military and those in Europe spent more.
But, if the Europeans spent more on the military, they would be spending more on their militaries, not on the US’s. US military spending would be cut under such a scenario. Cuts are implicit in the assertion that the status quo makes us worse off. If US military spending wasn’t cut as European military spending increased, then it makes no sense to say the current arrangement makes Americans worse off, because we would emerge no better off under the alternate scenario.
So: smaller US military, larger militaries in the other NATO countries. This means less breathing room to unilaterally launch major military actions. As a leftist and a critic of US imperialism, I, personally, would love this outcome. But somehow I think most of the grunting “America First” crowd wouldn’t.