To put it plainly: The U.S. and Russia, which together hold 90 percent of the world’s nuclear warheads, would be waging war against each other. And we aren’t talking about Cold War–style proxy conflicts like Vietnam and Afghanistan, but rather a red-hot conflict where American and Russian forces would be shooting and killing each other.

There are other questions to consider as well. Even if the U.S. did establish a NFZ (assuming World War III didn’t erupt as a consequence), how long would American aircraft be patrolling Ukraine’s skies? What exactly would be the objective? It might be a difficult query to answer, because NFZ missions tend to expand over time into more ambitious affairs. The U.S.-patrolled zones in northern and southern Iraq, imposed in 1991 and 1992 respectively, weren’t supposed to last over a decade — yet they did. The 2011 NFZ over Libya, endorsed by the U.N. Security Council, was originally tasked with protecting civilians. Yet that civilian-protection mission gave way to one of regime change very early on; driving Moammar Qaddafi from power became the top priority. While a hypothetical U.S. or NATO-enforced NFZ in Ukraine would obviously be different from the Iraq and Libya scenarios, countries that took part in it would run the risk of embarking on an operation that could last far longer than originally projected.