As shocking as was the sudden savagery the nation saw unleashed near the finish line of the Boston Marathon, it should not be surprising. Terrorism is part of life in these United States. As appalling as were the events in Boston, more remarkable is that the nation has not seen more of them.

There are many enemies, foreign and domestic, who are willing to massacre innocents, driven by twisted logic that such acts will somehow further their cause and exact some revenge for perceived injustices. The long list of potential motives for acting on that date in that place, and the diverse set of bad actors linked to those motivations, suggests how widespread is the threat our nation faces.

Monday was "tax day," the federal deadline for filing taxes, a focal point for anti-government, anti-tax groups. The attack also came just a few days before the April 19 anniversary of the 1993 assault by federal authorities on the Branch Davidian complex in Waco, Texas, which ended in the death of 80 cult members when fire erupted. Some extreme right-wing groups saw the Waco raid and the attempt to seize the group's weapons as a gross abuse of federal authority. The April 19, 1995 bombing of the Alfred P. Murrah Federal Building in Oklahoma City, killing 168, was a retaliation for the Waco siege, according to bomber Timothy McVeigh.

Mr. McVeigh was associated with the "patriot movement," a collection of various radical-right, largely rural militia and conspiracy-theorist groups who warn against growing federal authority and eroding Second Amendment gun rights. Monday was "Patriots Day" in Massachusetts.

Patriots Day would also provide motivation for an attack by Islamic terrorists. There is speculation that the Boston race could have replaced a planned attack on the New York Marathon, canceled because of Superstorm Sandy. Want more suspects? It was the birthday of Kim Il Sung, first leader of communist North Korea and grandfather of current dictator Kim Jong Un.

Then again, it might not be any of these groups or movements that carried out the attack, as new threats are ever emerging.

The success of law enforcement authorities in identifying and thwarting would-be terrorists has been impressive. Reviewing indictment records since 9/11, the New America Foundation, a nonpartisan public policy institute, found 50 right-wing extremists were caught with explosives or bomb-making components, 10 percent having obtained them through government sting operations. Twenty-three jihadist extremists were found with such components, roughly half as a result of stings.

But such violent fanatics must only succeed once to wreak havoc, as someone did Monday.

The challenge now is to identify the attacker or attackers and bring them to justice, to learn lessons that will help prevent future attacks, and to care for the afflicted and console those who have suffered the loss of loved ones.

The nation must move on with our games, our festivals, our celebrations of life, always cautious but never letting the fear terrorists seek to spread change who we are as a people. The Boston Marathon will return next year, new security precautions added, but with a defiant and joyful spirit that no one act, no matter how horrible, will stop a tradition beloved by so many.

President Obama put it well when addressing the nation Tuesday. Noting the exhausted runners who went to hospitals to give blood, the first responders and medical personnel who set aside fear of more bombs to attend to the maimed, the clergy who ministered to the distraught, the president said:

"If you want to know who we are, what America is, how we respond to evil, that's it. Selflessly. Compassionately. Unafraid."