The Economist: “A Vladimir Putin’s army closes in on Kyiv, and Russian rockets fall on Ukrainian cities, America and its allies are being asked to step in. On February 28th Volodymyr Zelensky, Ukraine’s president, repeated his earlier request for the West to “impose a no-fly zone over significant parts of Ukraine”. Some influential people agree. “Are we going to sit and watch while a world power invades and destroys and subjugates a sovereign nation?” asked Philip Breedlove, a former American general who commanded NATO forces until 2016, in an interview with Foreign Policy, a magazine. The idea of declaring airspace off-limits is an old one: after the first world war Germany was prohibited from any sort of military aviation at all, under the Versailles treaty. But the modern no-fly zone dates to the 1990s after Sadam Hussein, Iraq’s then dictator, attacked Kurds in the north of his country and Shias in the south. America, Britain and France declared no-fly zones in the northern tip of Iraq and over the bottom half of the country. To enforce it, they flew around 225,000 sorties between 1991 and 2003 (France pulled out in 1996). Similar no-fly zones were enforced by NATO over Bosnia and Herzegovina between 1993 and 1995, and over Libya in 2011, as part of a war that eventually toppled Muammar Qaddafi, the country’s leader.

No-fly zones prevent a country from using warplanes to attack military targets or civilians on the ground. But that humanitarian benefit comes at a cost. Simply declaring airspace off-limits is not enough. The power that declares it has to patrol the area with its own planes and be prepared to fire at enemy ones—four Iraqi planes were shot down in the 1990s. To conduct such patrols safely, it has to be confident its planes won’t get shot down. And that requires identifying, jamming or destroying air-defence systems on the ground. “The reality of a no-fly zone is [that] it is an act of war,” says Mr Breedlove…”