By Bob Casey Jr.

When the 56th Stryker Brigade of the Pennsylvania National Guard returned from Afghanistan in 2009, they came home without Specialist Chad Edmundson from Williamsburg, Pennsylvania.

U.S. Senator Bob Casey Jr.

Specialist Edmundson was one of more than 3,000 service members killed and 30,000 wounded by improvised explosive devices over the past decade.

Improvised explosive devices, or IEDs, are the deadliest weapon our military men and women face on the modern battlefield.

Hidden in trash, under bridges, and in the folds of clothing, they haunt our military's footsteps and tire treads in countries like Afghanistan and Iraq.

The U.S. Military has spent billions of dollars trying to combat them - with good success - but IEDs remain a grave threat.

Long before the 56th Stryker Brigade came home from its deployment, I had been working on this issue. But Specialist Edmundson's story inspired me, and I knew we had to do more.

So I held hearings in the Senate Foreign Relations Committee and introduced legislation calling on the governments of Pakistan, Afghanistan, and other Central Asian countries to help stop fertilizers used to make bombs from getting into the hands terrorists.

I visited Afghanistan and Pakistan and urged politicians and business people alike to crack down on the illegal sale of these fertilizers.

And I worked with the Defense Department to streamline and coordinate our research into lifesaving technologies that our military men and women could use in the field.

Then, in 2011, I led nineteen of my Senate colleagues in writing a letter to then-Secretary of State Hillary Clinton, requesting her help on this important issue.

Since becoming Secretary of State, she had focused intensively on measures to protect our military men and women in the region. Everyone who signed that letter had sat at too many hospital bedsides.

We had seen too many families torn apart by this menace. We had to do something, and do it quickly.

Secretary Clinton had a long history of supporting our Veterans and our military men and women. As a senator, she worked to help Veterans suffering from PTSD get the resources they needed.

With former Sen. Christopher Dodd, of Connecticut, she introduced legislation to support families of wounded Veterans while they cared for their loved ones.

She fought tooth and nail to improve services at the Department of Veterans Affairs.

And sure enough, soon after we wrote that letter, Hillary Clinton answered our call.

Over the next few years, Hillary worked tirelessly to make sure that these fertilizers were not sold to the wrong people. She met countless times with our partners in the region.

And by the time she left the State Department in 2013, Pakistan had begun working with U.S. experts to stop the sale of fertilizers near the Afghan border and to dismantle the terrorist networks that smuggled these dangerous chemicals.

Her work and that of a number of U.S. diplomats saved American lives. And I'm proud to have worked with her to advance this cause.

This Tuesday, I'll be voting for Hillary in the Democratic primary.

Hillary has a long record of fighting for important priorities like raising middle-class incomes, fighting opioid addiction, and increasing access and funding to early childhood education.

And of course, her commitment to our fighting men and women and our Veterans is unwavering.

Our service members are some of the bravest, most committed people to walk the earth. Undaunted, they serve with integrity and courage. Each and every one of them sacrifices so much to make us safe. Some of them give, as President Lincoln once said, "the last full measure of devotion" to their country.

As public officials we must ensure that our work is worthy of the extraordinary valor of our service members.

We cannot let these brave men and women down.

Whether embarking on their fourth deployment or descending into a combat zone for the first time, we need to ensure that they are properly trained and equipped.

And when they return home, with scars both seen and unseen, wanting to return to civilian life, we need to ensure they have the access to the care and opportunities they so richly deserve.

As president, Hillary Clinton will do just that, and repay part of the debt we owe our service members and our families.

U.S. Sen. Bob Casey, Jr., a Democrat, is Pennsylvania's senior United States Senator. He writes from Washington D.C.