Donald Trump shocked foreign-policy professionals and observers when he remarked to The New York Times that if he were president, the United States might not come to the defense of an attacked NATO ally that hadn’t fulfilled its “obligation to make payments.” The remark broke with decades of bipartisan commitment to the alliance and, as Jeffrey Goldberg wrote in The Atlantic, aligned well with the interests of Russia, whose ambitions NATO was founded largely to contain. One Republican in Congress openly wondered whether his party’s nominee could be “seemingly so pro-Russia” because of “connections and contracts and things from the past or whatever.” It’s not unlike Trump to make shocking statements. But these ones stoked particular alarm, not least among America’s allies, about the candidate’s suitability for the United States presidency. So what’s the big deal? What does NATO actually do?