The vast majority of which owe their national security to the US. Security which we are not properly compensated for. Trump will force our lazier "allies" to get their houses in order and shoulder more of the burden for protecting themselves.
The United States pays only 22.1% of the direct costs of NATO- about $500 million annually. (Germany pays 14.6%, France pays 10.6%, the UK pays 9.8%, Italy pays 8.4%, Canada pays 6.6%, Spain pays 5.8%, Turkey pays 4.3%, and the remaining 20 NATO allies- the Netherlands, Luxembourg, etc.- pay about 1% each on average.) On a per capita basis, the U.S. spends considerably less on NATO than either Germany or France.
The indirect costs of NATO, of course, are somewhat higher, as you'd naturally expect when a country voluntarily spends 54% of its discretionary budget on defense. That's totally nuts, and the way to address it would be to simply reduce military spending. But that's obviously not what he's proposing, since he knows it would get him booed off the stage. He's talking about keeping the military budget at present levels, and instituting a shakedown of NATO allies with an explicit threat to disregard our treaty obligations if countries don't pay up and are subsequently attacked. (The NATO treaty has been ratified by a Congressional supermajority, so this would be unconstitutional, but Trump insists that he can "negotiate" his way out of it and get a "better deal"- albeit one that could not possibly net us any more than $1 billion.)
Regardless of whether he intends to follow through on these statements or not, the mere fact that he's describing NATO in public as a protection racket instead of a treaty has already undermined national security. It's not as if both our NATO allies and Putin can't hear these rants (via "our Internet"), and the leaders of several of these countries have already expressed their suspicions that the United States might be willing to abrogate its treaty obligations if this fool gets elected. In fact Trump is making it clear that there could be no point in signing a treaty with the United States at all. It's now obvious to the rest of the world that the American political system has reached an ominous level of instability which is possibly sufficient to elevate a cretin like this to the presidency.