In an article published a year ago, Brian Rathbun looked at the views held by U.S. liberals and conservatives just after World War II about the institutional design of the UN and NATO.* Rathbun argued -- I'm simplifying for the sake of brevity -- that liberals (mostly though not exclusively Democrats) wanted a more co-operative, multilateral approach and strategy because they were more "trusting" of allies and less fearful that allies would take advantage of the U.S., whereas conservatives (mostly though not exclusively Republicans) were less "trusting." Thus on NATO, for example, the Truman administration, Rathbun writes, "was willing to provide a guarantee of European security before the Europeans could effectively contribute to the alliance because it expected future reciprocity." By contrast, conservative Republicans wanted a "unilateral declaration of American intent," a sort of Monroe Doctrine for western Europe, rather than NATO, "but even moderate Republicans wanted the Europeans to first demonstrate their commitment to continental defense before the conclusion of any pact...." The less "trusting" Republicans feared 'free-riding' (in the non-technical sense of that phrase), i.e. they feared that the European states in NATO would not contribute adequately to their own defense.
Rathbun is interested in making a theoretical argument about social psychology, trust, and dispositions to co-operate, so he doesn't, at least from what I gather based on a perusal of the article, ask which side turned out to be right. Were Republicans correct to fear that allies would take advantage of a U.S. commitment to their security and not contribute to their own defense? It depends, I suppose, on how strong a version of the argument one takes. NATO members certainly have maintained their own defense budgets and militaries, but the question of relative contributions has been a sore point in recent years and probably throughout much of the alliance's history. And when it comes to U.S. security commitments to allies in Asia, the situation is probably etched in sharper relief. Robert Kelly (who, like Rathbun, blogs at Duck of Minerva) has pointed out that the level of defense spending by South Korea is "irresponsibly low," i.e., South Korea is taking advantage of the U.S. security umbrella to avoid spending very much on its own defense. Trust is all well and good, but in these contexts it does seem to have led to what IR scholars loosely (because it's not really the technical definition) call free-riding. I never, ever thought I'd quote Reagan with approval about anything, but here one can't help recall the slogan -- admittedly taken out of context -- "trust but verify."
*B. Rathbun, "The 'Magnificent Fraud': Trust, International Cooperation, and the Hidden Domestic Politics of American Multilateralism after World War II," Int'l Studies Quarterly v.55 (2011):1-21.