This past week, Dzhokhar Tsarnaev reportedly told investigators that he and his brother set off bombs near the finish line of the Boston Marathon in part because of their opposition to the U.S. wars in Iraq and Afghanistan. As a Marine who fought in Afghanistan in 2008 and 2010, the news made me wonder: Had my war brought the horrors of battle home?

My family's home is a few blocks from the blast site, where flying shrapnel killed three people and severed many limbs. When a relative told me that he wanted to kill those responsible, I couldn't help feeling that I had somehow failed. My family sounded like any of the Marines I'd met after a comrade stepped on an improvised explosive device: angry, confused, spiteful. War had seeped through my front door, and now my five-foot-tall flower child of a mother wanted revenge served cold.

When the Sept. 11, 2001, attacks happened, I was a 13-year-old boy barely out of my Pokemon phase. I remember that day vividly, as we all do: gigantic pieces of steel dissolving into a sea of ash.

By April 15, 2013, my frame of reference had changed. Now, at 25, I've experienced the horror of the Boston finish line many times. In Afghanistan, I've shoved my knee into wounded Marines' pressure points, smelled the cordite of gunfights and explosions, and said goodbye to dear friends. Recently, however, I've returned home to complete an English degree at Georgetown University.

The images of the Boston bombing reminded me of things I saw in Helmand province, not the streets where I do my Christmas shopping. Many witnesses described the marathon carnage as "a war zone," and indeed it was: mangled flesh, shocked faces, splattered blood.

Except the runners and spectators in Boston weren't wearing body armor and helmets. They weren't combatants. Rather, they were strangers, family members, co-workers and friends in Nikes and New Balances, turning sweat-drenched T-shirts and belts into tourniquets.

I deployed to Afghanistan believing my presence in that country would help stop attacks such as Boston's from happening. But instead, my war had spilled over.

The Tsarnaev brothers aren't the first terrorists to cite U.S. military intervention as a reason for targeting civilians, and they won't be the last. Despite our best efforts and valor, I wonder, have America's wars made the homeland less safe? Sure, we've killed and captured thousands of radicals who wanted to harm Americans. But in doing so, have we created more?

It wasn't always easy to justify serving in a war that has devolved from its initial aim of ousting the Taliban and al-Qaida to a nation-building effort that appeared to have come 10 years too late. The conflict has dragged on for more than a decade, becoming increasingly unpopular after years of mixed results and no clear definition of victory. The counterinsurgency mantra of "clear-hold-build" echoed in our ears as we fought an elusive enemy and slowly pushed it out of the city centers. Day by day, we measured victory by the number of wells we had helped build and the time that had passed without a casualty.

Some of my best friends came home in flag-draped coffins, and no one ever convincingly explained to me why and what for. On a recent winter afternoon, after Afghan President Hamid Karzai delivered an upbeat speech at Georgetown on the future of Afghanistan, I had the chance to ask him what the sacrifice of my brothers-in-arms meant to him and his countrymen.

The answer I received was a diatribe. Karzai cited Sept. 11, 2001, and America's global war on terror but never directly answered my question. I would have liked a "thank you" or a sentence with "greatly appreciated" in it. But there was not a hint of gratitude in his response.

While deployed, I went to bed at night believing I was protecting the homeland because coming after my fellow Marines and I was a much easier commute for those hellbent on killing Americans. That argument no longer makes sense if my war has inspired our enemies at home.

I'd like to believe that my war prevented an attack such as Boston's for some years. If the 16 months I spent in Afghanistan delayed the bombing for just a day, then it would have been worth it.

But my war failed to help those people at the finish line. As those bombs exploded, my war came home.

Thomas Gibbons-Neff, now a sergeant in the Marines, is president of the Georgetown University Student Veterans Association. He served as a rifleman in the 1st battalion, 6th Marines in Afghanistan in 2008 and 2010. He wrote this for The Washington Post.