Both the United States and Russia last month pulled out of the Intermediate-Range Nuclear Forces Treaty (INF), a Cold War–era pact that prohibited land-based ballistic or cruise missiles with ranges between 311 and 3,420 miles. That agreement limited just one class of weapons, but it is not the only accord poised to end: The much-broader New Strategic Arms Reduction Treaty (New START) will expire on February 5 next year, unless both parties agree to extend it—which they may not do.

New START limits the number of missiles the U.S. and Russia deploy, with an eye toward reducing the overall number of nuclear weapons in the world. Without it, for the first time since 1972 there would be no limit on how many warheads either nation can build and deploy. As tensions rise, both countries are looking to modernize their nuclear weapons, and Russia in particular is teasing terrifying new missiles that—if they work—could bypass the U.S.’s elaborate system of ground- and satellite-based defenses.

“The Russians really hate missile defense,” says Jeffrey Lewis, a nuclear policy expert and professor at the Middlebury Institute of International Studies in Monterey, Calif. “They really don’t like the possibility that they might be outmatched technologically. So there’s a whole battery of Russian programs—from the doomsday torpedoes, to nuclear-powered cruise missiles, to hypersonic reentry vehicles, to anti-satellite weapons.”