As events in Boston proved so tragically, you can take extraordinary precautions to deter terrorism, but you won't always succeed.
But that doesn't mean the precautions aren't important. During the years since the attacks on New York City and Washington, D.C., a great many terrorist plots have been foiled and lots of innocent lives have been spared as a result.
By the same token, the gun control legislation proposed in the Senate this week would not have deterred every act of gun violence. It was a pretty tame piece of legislation, one that would make it more difficult for the felons and murderers who can now purchase weapons at gun shows to get their hands on the guns they will later use to kill the innocent.
More than nine in every 10 Americans supported the measure. The National Rifle Association opposed it.
The senators had a simple choice to make: Take this one little step to protect the next group of children from being massacred or to side with those who get rich pandering for guns.
That's the choice each senator had to make.
I'm talking about you, Ron Johnson. You had that choice to make and you made the wrong choice.
I know I disagree with your general political stance, but that's irrelevant. You can be a right-wing Tea Party Republican and still choose to side with children and not the gun lobby. But you didn't choose to do that and your vote will be an albatross around your neck until the day you die.
The vote was taken on a day when all of us, Democrat and Republican, liberal and conservative, were marveling at the courage of the people in Boston who, as soon as the bombs detonated, ran back into the face of danger to assist the wounded.
The United States Senate couldn't muster that courage. Ninety percent of Republicans and 10 percent of Democrats couldn't even take the first tiny step to protect the innocent from being murdered. As President Obama said, it was a "shameful day in Washington."
It won't be the last day. It may not even signal the last defeat. But sooner or later these cowards won't be able to live with their consciences any longer and they will do something about this attitude of darkness that can look terrified children in the face and walk away unmoved.