Such explicit prohibitions are critical, because the entire aviation system works on the premise that unless airspace is marked as off-limits, it is presumptively safe and legal for flight. The airlines want to minimize cost and time by going as directly as possible, and they rely on regulators to tell them where they cannot go.

For an example of how a system functions when it is built on a different premise, consider Chinese aviation. A vast majority of Chinese airspace is military-controlled and usually unavailable to airlines. Thus flights in China take much longer, burn much more fuel and are delayed much more frequently than those in North America or Europe.

Before Malaysia Airlines Flight 17 took off on Thursday, its crew and dispatchers would have known that a few hours earlier Ukrainian authorities had prohibited flights at 32,000 feet and below across eastern parts of their country, “due to combat actions ... near the state border” with Russia, as the official notice put it, including the downing of a Ukrainian military transport plane earlier in the week.

Therefore when they crossed this zone at 33,000 feet, they were neither cutting it razor-close nor bending the rules, but doing what many other airlines had done, in a way they assumed was both legal and safe. Legal in much the way that driving 63 in a 65-mile-per-hour zone would be.

And safe, not just for regulatory reasons, but because aircraft at cruising altitude are beyond the reach of anything except strictly military antiaircraft equipment. During takeoff and landing, airliners are highly vulnerable: They are big, they are moving slowly and in a straight line, they are close to the ground. But while cruising, they are beyond most earthbound criminal or terrorist threats.

This is why, even during wartime, airliners have frequently flown across Iraq and Afghanistan. The restricted zone over Ukraine was meant to protect against accidental fire or collateral damage. It didn’t envision a military attack.

After each crash, disaster or terrorist episode, it is natural to point fingers and say, Why didn’t we foresee that specific threat? Thus one attempted shoe bombing leads to a decade of shoes-off orders in security lines. The truth is that air transportation, like most other modern systems, could not operate if it fortified itself against every conceivable peril.

Malaysia Airlines, its crew and passengers and the civil aviation system are the objects of this crime and tragedy. The finger-pointing should not be at them, but at the criminals.