Only twice since 1776 has the United States sustained surprise attacks at home.

The second attack was Sept. 11, 2001, when hijackers commandeered commercial aircraft on morning flights and steered two into the twin towers of the World Trade Center in New York and a third into the Pentagon just outside Washington, D.C. A fourth was destined for the U.S. Capitol until passengers, realizing what was about to happen, chose to fight back and sacrificed their own lives as United Flight 93 speared into the ground near Shanksville, Pa.

The first attack was 60 years earlier and 67 years ago today, also a clear and bright morning. The people of Pearl Harbor, Hawaii, a U.S. territory on Dec. 7, 1941, also expected nothing out of the ordinary.

The rest of the world was in turmoil, but President Franklin Delano Roosevelt had been elected time and again on a pledge to keep America out of the war initiated across Europe, North Africa and parts of Asia by Adolph Hitler and the fighting under way since 1937 between Japan and China.

Suddenly, however, on that “day that will live in infamy,” the United States was drawn in and, with a unity of purpose unprecedented in national history, spent years defeating enemies on two fronts.

That unity of purpose, for myriad reasons, did not follow the attacks of 2001. For one thing, the enemy is different. The war on terror is not about real estate. It’s about ideology. And that makes for a very different situation.

But both attacks do serve as reminders that we live in a world where the global situation can, as the saying goes, change without notice.

It would be unrealistic and counterproductive to live our lives in fear. Too many struggled too long in too many places for us not to enjoy the security their efforts have provided.

But by remembering what happened on Pearl Harbor day and on 9/11, we should know better than to take anything for granted.