Terrorism is the systematic use of violence to create a climate of fear. There is no question that what happened yesterday in Boston meets this definition. Two bombs near the finish line of that city's fabled marathon did what they were intended to do: Kill, maim, and frighten. The death of innocents — and innocence, if we had any of that left — is an important ingredient in this brand of evil, and among yesterday's victims was an 8-year-old boy from Dorchester, Mass., who had just hugged his father at the finish line. His sister had a leg blown off. Their mom was badly injured, too.

But the true power of terrorism extends far beyond the physical blast range of a few bombs and the shattering of a relatively small number of lives (tragic, of course, as the latter is). Thanks to TV, Twitter, and mobile phones, the actual impact of terrorism — fear and insecurity — was felt nationwide within minutes. Cops swarmed Times Square. The Secret Service extended its security perimeter around the White House. On buses and trains across the country, rush-hour commuters were reminded to be on the lookout for anything — or anyone — suspicious. "If you see something, say something," as the mantra of the Department of Homeland Security goes.

Whoever was behind yesterday's attack killed a few, wounded scores, and scared millions. There's a term for this: asymmetrical warfare. An opponent with inferior resources, manpower, and technology can nevertheless inflict disproportionate damage on an enemy. The ultimate example of this concept, of course, was September 11, 2001, when 19 hijackers spending just $500,000 — well, you know what they did. But the true impact the 9/11 hijackers had went far beyond the physical damage they inflicted that day. September 11 caused us to burn through trillions on the two longest wars in our history, vastly expand our national security and intelligence bureaucracy, and give up a portion of our civil liberties.

And yet for all that: Do you feel safer? The notion that we "beat" Osama bin Laden is debated by some who argue that he achieved his true goal: inflicting huge damage on the U.S. economy, and making us feel insecure. The complacency and safety that we took for granted on September 10, 2001 has never returned, and what happened yesterday was a reminder that it is never likely to. In this respect, have we really won? We're forced, years later, to take off shoes, endure patdowns and surrender a too-big tube of toothpaste before we board a flight. That's the result of asymmetrical warfare.

Then there is the absolute randomness of terrorism: A marathon in Boston. A movie theater in Colorado. Restaurants, trains, schools, shopping centers, hospitals, on and on — they're all wide open. What happened yesterday could have happened anywhere at any time. And it still could.

What more can we do? "We will bankrupt ourselves in the vain search for absolute security," Dwight Eisenhower once said. Ike's observation has even more resonance today. How much more can we spend on security? We're $16 trillion in the hole now. But there's another type of bankruptcy: the loss of individual liberty over the past 11+ years as part of the government's broader effort to maintain security. That we have come to accept this — the relinquishing of a portion of what the founding fathers bequeathed us — is a form of surrender.

We don't know who was behind yesterday's outrage. And for all the speculation about Islamic extremists, we must remember that domestic terrorism is also a possibility. This, after all, is the third week of April, a reminder of several homegrown horrors:

* April 16: Massacre at Virginia Tech University (2007)

* April 19: Oklahoma City bombing (1995)

* April 20: Massacre at Columbine high school (1999)

Then there's this: Yesterday was Patriots' Day in Massachusetts. Always held on the third Monday in April, it commemorates the battles of Lexington and Concord in 1775 — the first shots fired in our Revolutionary War. Perhaps it's only a coincidence that yesterday's attack occurred on Patriots Day. Or perhaps not.

After all, the anti-government "Patriot" movement is one of the fastest growing internal security threats in the country. Since 2009, there has been a sharp increase in these so-called "militia" or "patriot" groups. The Southern Poverty Law Center, which tracks these groups, says they are fueled by "changing demographics driven by immigration, the struggling economy, and the election of the first African-American president."

In 2012, the SPLC identified 1,274 such groups, and says they are generally engaged in "groundless conspiracy theorizing, or advocate or adhere to extreme anti-government doctrines." Some, the SPLC says, "advocate or engage in violence," many "warn of impending government violence or the need to prepare for a coming revolution." Some, it adds, are racist (including some African-American extremist groups who hate whites). Some think the government is planning to round up citizens and place them in concentration camps. Still others think they can set their own laws and shouldn't have to pay taxes (yesterday was tax day, after all).

As a grim President Obama vowed last night, we'll find out who the coward — or cowards — behind yesterday's outrage was. We always do. Meantime, this advice: Fear not. Stay alert, but fear not. That's how we'll win. Keep calm — and carry on.