AMNA NAWAZ: Tonight, we begin a series looking back at the U.S. invasion of Iraq 20 years later.

The attacks began March 20, 2003, in a thunderous hail of airstrikes on Baghdad.

Soon, American troops would race across the desert from Kuwait toward the Iraqi capital.

Special correspondent Mike Cerre was embedded with the Marines of Fox Company, 2nd Battalion, 5th Marines, known as Fox 2/5.

They would help take Baghdad and return to Iraq on subsequent deployments.

Mike has stayed in touch with many of those Marines and their families.

And, tonight.

he begins our coverage, which will stretch over the coming weeks, with this look at Fox 2/5 then and now.

MIKE CERRE: Still teenagers 20 years ago, most of these Marines enlisted right after 9/11, straight out of high school, for what became the largest military operation in the war against terror.

WOMAN: My husband said, it seems like every generation has to do this, and this is this generation's battle.

MIKE CERRE: Do you think the rest of the country fully appreciates... WOMAN: No, they don't have a clue.

MAN: And we got an old, beat-up helmet for you, which I'm going to mess with a little bit.

And we're going to fix it up on the plane over there.

MIKE CERRE: OK. As a reporter, and a Marine veteran of Vietnam, I embedded with Fox Company, 2nd Battalion, 5th Marines in 2003, both to report on the war for ABC News and to chronicle how much another generation of Americans would likely be changed by a war.

As we're now moving and trying to penetrate into Southern Iraq as fast as we can.

PETER JENNINGS, ABC News: As you hear from Mike Cerre, it is now a full-scale invasion.

MIKE CERRE: Any abstract notions of war these Marines might have had quickly turned into their reality of war, as one of the first American units to invade Iraq in March 2003.

SOLDIER: We came out with 206.

I want to go home with 206.

If we don't have to get decisively engaged, that's fine with me.

MIKE CERRE: First Sergeant Ed Smith was Fox 2/5's senior enlisted man and one of the few with previous combat experience.

He delayed his scheduled retirement to accompany his unit to Iraq.

On their final push to Baghdad, First Sergeant Ed Smith was Fox 2/5's first and only fatality during the unit's first of several deployments to Iraq over the next five years.

SHELBY SMITH ROBINSON, La Mesa Police Department: I think it affects me more now in my adult life than it did obviously when I was 8, because I couldn't really process it, being so young.

MIKE CERRE: His daughter Shelby Smith and other Gold Star families who lost a relative in Iraq don't need anniversaries to remind them of the sacrifices made in Iraq.

SHELBY SMITH ROBINSON: But for people who do not know it personally, and it hasn't affected them personally, I think the war probably never crosses their mind.

MIKE CERRE: Shelby Smith is continuing her fathers legacy as a police officer, like the new career he had already started prior to his delayed retirement from the Marines.

SHELBY SMITH ROBINSON: Twenty years is a long time to heal and to process things.

There's times at work I feel close to him there, because I imagine him doing the same things that I'm doing.

And I can feel him next to me sometimes.

I think he's keeping me safe.

I'm really happy to have been able to kind of follow in his footsteps.

MIKE CERRE: Others I have reconnected with over the years have had to deal with less obvious injuries.

MIKE ELLIOT, Iraq War Veteran: Don't sweat the small stuff, Mike.

This is what started the whole... MIKE CERRE: I first met former Corporal Mike Elliot reading this pop psychology book for teenagers while in his foxhole wearing a chemical protection suit.

MIKE ELLIOT: This was some bathroom material at home.

It's called "Don't Sweat the Small Stuff."

And the main part, simple ways to keep yourself cool in stressful times.

MIKE CERRE: Michael Elliot was one of the first members of the unit to enter a Veterans Administration residential program for PTSD.

The VA believes more than one in four Iraq War vets have or will experience varying degrees of PTSD.

He has since earned a counseling degree and has helped other Montana vets deal with their PTSD through wilderness programs.

SAL CHAVEZ, Iraq War Veteran: About a week ago, I had a sniper get real close to me.

You never know out here.

An IED, as we have seen -- I guess, the first couple of months, we them every day.

MIKE CERRE: Former Navy Corpsman Sal Chavez, Fox 2/5's medic, has tried some of the newer alternative PTSD therapies to help him deal with some of his most intense Iraq memories.

This is a virtual reality reenactment of a roadside ambush when their 1st sergeant was killed.

SAL CHAVEZ: Everything was on fire.

It was loud, rounds being shot off, main tech gun being shot off.

You could feel the percussions of all of the explosions.

MIKE CERRE: How do you feel in your body right now watching this?

SAL CHAVEZ: Just a little intense.

So I had to find it within me and start making the hard choices of facing myself, finding help, doing whatever I had to do to get healthy again.

MIKE CERRE: Others have not as been successful.

More members of Fox 2/5 have since died from suicide than were killed in combat.

All three have been diagnosed with PTSD.

MAN: God love you.

We knew you were going to make it.

MIKE CERRE: Since their homecoming after the 2003 invasion, most of these Marines have resumed their lives interrupted by war and now have families and civilian careers.

RYAN SMITH, Iraq War Veteran: It did help us that the country was unified behind us.

We felt after 9/11 that everyone wanted to go and get the bad guys, and that we had a lot of support from our country, which is way different than my father-in-law and my uncle, who served in Vietnam.

MIKE CERRE: Former Sergeant Ryan Smith learned of his acceptance to college in his first letter to home after reaching Baghdad in April 2003.

RYAN SMITH: I never really thought I had a chance of getting in, but, hopefully, we get out in time for me to start in September.

MIKE CERRE: He went on to UCLA Law School and is now a corporate attorney near Newport Beach, California.

RYAN SMITH: So, after getting out of the Marine Corps, I struggled making social connections.

I struggled interacting with people that hadn't served in the military.

I felt I had a chip on my shoulder, that the average civilian couldn't understand what me and my friends had gone through.

MIKE CERRE: As far removed from war as his personal and professional life are today, some memories from his Iraq experience are still inescapable.

MAN: We have got a vehicle coming.

MAN: We have got a vehicle coming.

MAN: We got a tracer?

MIKE CERRE: The same day that their 1st sergeant was killed, the unit was involved in a civilian tragedy at a roadblock that killed 10 Iraqis, some of them children.

It was later determined to be a fog of war accident.

RYAN SMITH: I think yesterday was probably a defining moment that I have had so far in my life.

And after yesterday and the day before, I think there's no more curiosity.

I think people have seen the horrors of combat, and no one looks forward to any more of it.

MIKE CERRE: I can remember coming up to you that morning afterwards, and you were certainly a bit rocked back on your heels, and you said you had had enough of war.

RYAN SMITH: You didn't think about innocent people and children getting in the line of fire.

And once that happened, it shattered whatever noble vision I had of war or romanticized vision of war.

And it really dawned on me that war is a dirty, ugly business.

MIKE CERRE: Did your feelings about the war change while you were in it, or did take a period of time to evolve afterwards?

RYAN SMITH: A lot of us took a more nuanced approach towards the war after that and realizing that we did what we were told to do, and we served our country, and were proud that we stood up when we were asked to serve, and that we stood shoulder to shoulder with some of the best people I have ever met in my life and some of the finest Americans.

But, at the end of the day, you look how the Iraq War ended, and it does make you question, what was -- what was that all for?

MIKE CERRE: Whatever their personal feelings about the war 20 years later, they all share what former chief Justice Oliver Wendell Holmes called the incommunicable experience of war, when their hearts were touched by fire.

For the "PBS NewsHour," this is Mike Cerre.

AMNA NAWAZ: And, tomorrow, we will continue our look at the war and its aftermath through the eyes of two Iraqi families.