WASHINGTON — THE acid that corroded George W. Bush’s presidency was fear — spreading it and succumbing to it.

You could see the fear in his eyes, the fear that froze him in place, after Andy Card whispered to W. in that Florida classroom that a second plane had crashed into the twin towers.

The blood-dimmed tragedy of 9/11 was chilling. But instead of rising above the fear, W. let it overwhelm his better instincts. He and Dick Cheney crumpled the Constitution, manipulated intelligence to go to war against a country that hadn’t attacked us, and implemented warrantless eavesdropping — all in the name of keeping us safe from terrorists.

Americans want to be protected, but not at the cost of vitiating the values that make us Americans. That is why Barack Obama was so stirring in 2007 with his spirited denunciations of W.’s toxic trade-offs. The up-and-coming senator and former constitutional law professor railed against the Bush administration’s “false choice, between the liberties we cherish and the security we provide.”