We reached a new low this week when one of the two major presidential candidates called into question our U.S. treaty obligation to NATO. Donald Trump’s remarks in a New York Times interview that he would only defend the Baltic states against a Russian invasion if they “have fulfilled their obligations to us” leaves open the question of whether under a President Trump the United States would honor its Article V commitment to treat an attack against one NATO member as an attack against all. Our obligation, and that of the Baltic states, is to come to the assistance of any NATO ally, without conditions.

Trump told the Times he had a condition – that the United States would be “reimbursed.” This “obligation” he refers to is the relatively recent agreement among NATO members that they would each spend at least 2 percent of their GDP on defense. The United States has long sought to persuade our allies to pick up more of the cost of modernizing, deploying, and fighting. But the 2 percent is a political target. Currently, only 5 of 28 NATO members meet the target and, indeed, the next president will have to work harder to convince European capitals to increase defense spending. Success, however, is more likely through diplomacy, and not bullying. And it is more likely to fail if the U.S. government questions the foundation of NATO: the Article V commitment of the 1949 Washington Treaty. Collective security works only when all countries believe everyone is committed to the common defense. And paying 2 percent of GDP to NATO is not a treaty obligation; defending each other is.

Trump’s statements run counter to international law and commitments, counter to the very status quo that helps America prosper and keeps us safe. It questions the foundation of our only operational collective security alliance, an institution that has served us well in many ways, starting from keeping the peace in the aftermath of World War II and through the Cold War. This was no small feat. We deterred the Soviet Union with conventional and nuclear NATO forces until under Gorbachev the USSR changed its approach to the West and ultimately ceased to exist.

But beyond that, NATO continued to bring stability to Europe by motivating former Warsaw Pact members who were eager to join NATO to resolve their disputes with neighbors, become more democratic, and adopt NATO norms and operating principles. This resulted in the NATO expansion that now protects the Baltic states from a Russian invasion like those outside of NATO in Ukraine in 2014 and in Georgia in 2008 where Russia still holds 20 percent of Georgia’s territory. In the spirit of fostering European stability, NATO conducted two humanitarian interventions in Bosnia and Kosovo.

And let us not forget the war on terrorism. The attack on the United States on Sept. 11, 2001, was the one and only time NATO invoked Article V. NATO declared that an attack against the United States obligated the other – at the time 19 nations – to action. In early October 2001, five NATO AWACs surveillance aircraft joined the U.S operations protecting U.S airspace after the attacks. NATO then deployed a naval force to the eastern Mediterranean and provided access to European airspace, airports, airfields and intelligence. At the time, President George W. Bush said, “This has never happened before, that NATO has come to help defend our country, but it happened in this time of need and for that we are grateful. Together, we’re building a very strong coalition against terror, and NATO is the cornerstone of that coalition.” NATO later deployed to Afghanistan with us, out of Europe.

Related: Defense One's complete coverage of NATO.

NATO is worth the cost. We have countered threats as an alliance of 28 members today (soon to be 29 with Montenegro’s accession) effectively and increased the stability in Europe that undergirds our political and economic relationship with our number one trading partner, and also makes it possible for us to take military action from European bases and with European allies in the Middle East and elsewhere.

But today Trump’s remarks make us less safe because, regrettably, one of the major threats facing NATO is the violation of the sovereignty of eastern member states by Russia. Putin’s Kremlin has made it clear that it regards NATO as an adversary. Moscow seeks a buffer zone of subservient states between it and Western Europe, potentially to include NATO member states. It certainly includes those states outside NATO like Ukraine and Georgia who have asserted the right for their people to pick their alliances and associations. The Russian government has repeatedly proposed dividing Europe, in a 19th century formulation, into two spheres of influence – one Russian and the other U.S. and Western European.

Trump’s refusal to acknowledge our treaty commitment to the Baltic states

could have the same permissive effect on Russia as Dean Acheson’s comments on January 12, 1950 when he failed to include South Korea in America’s area of interest, or “defense perimeter.” In that case, Stalin and Kim-il-Sung of North Korea were likely emboldened to invade South Korea. Likewise, Trump’s irresponsible commentary this week could well embolden Putin to test Article V now, soon, or certainly if there is a Trump president (God, please, forbid). In any event, Putin can now sit back and watch Trump do the work Putin set out for himself – destroying NATO, transatlantic, and European unity.