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David L. Ryan / The Boston Globe via AP

Medical workers aid an injured man at the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. Two bombs exploded near the finish of the Boston Marathon on Monday, killing at least two people, injuring at least 22 others and sending authorities rushing to aid wounded spectators.

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David L. Ryan / The Boston Globe via AP

People react as an explosion goes off near the finish line of the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions went off at the Boston Marathon finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts.

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Winslow Townson / AP

An unidentified Boston Marathon runner leaves the course crying near Copley Square following an explosion in Boston Monday, April 15, 2013.

We didn't need to be reminded that even here in the U.S., an ocean away from the turmoil and volatility of North Africa and the Middle East, we are vulnerable. We received that reminder anyway Monday, when two explosions seconds apart shattered the festive atmosphere of the Boston Marathon, killing at least three people and injuring, by some reports, nearly 150. It was almost certainly an act of terrorism.

It's hard not to regard the weekly, sometimes daily reports of carnage in Syria or other troubled countries with a mixture of weariness and relief. Sometimes we can't help but find a measure of solace in the belief that we are, but for a few awful exceptions, somehow immune to terror on these shores. And then something like the Boston Marathon bombing slaps us back to reality, reminding us that our invulnerability is an illusion.

Apart from what we can only hope will be a relentless quest to apprehend the perpetrators, there is really only one meaningful lesson to take from this: No nation is safe from the targeted terror of political violence, whether it be domestic (Monday was, after all, tax day) or external (Boston was the jumping-off point for al-Qaida's 9/11 attacks and the city where the idea of a strong and independent America first got its footing).

That doesn't mean terror is an inevitable consequence of living in today's society and that we are helpless to stop it. On the contrary, it is a reminder that vigilance is a full-time job for everyone, not just law enforcement. The world is full of people who don't like us, and as the recent shootings of two Texas prosecutors and a Colorado prison official demonstrate, some of them are homegrown.

Get ready for the inevitable debate between security and privacy, between surveillance and freedom. The line is a fine one, and it moves with the flow of national discourse. That is our world today -- and it will be our world for some time to come.