So it was on Sept. 11 for Maj. Wally Powers, of the Second Battalion, 25th Regiment of the 4th Marine Division - as storied a unit as any in the Corps.

Just as it was for Cpl. Sean Tallon, also of the 25th, who died at Ground Zero.

Powers is reserve officer - a bit on the short side for a major of Marines, but a commanding presence nevertheless.

In civilian life, he is a New York City firefighter, assigned to Engine 45 in The Bronx.

"Just a firefighter," he said last Wednesday, the self-deprecation easily defeated by the twinkle of fierce pride in his eyes.

But it will probably be a while before Powers returns to the firehouse - to life as it was before 9/11.

Before Operation Enduring Freedom, before the call to active duty that led the major and his men first to Camp Lejeune, N.C., thence to Norfolk, Va. - and then, last Wednesday afternoon, into New York Harbor aboard Iwo Jima.

How bittersweet the moment for the men of the 2nd Battalion, 25th Regiment.

It was a homecoming.

They're New Yorkers, for the most part, and they stood shoulder to shoulder and ramrod straight (along with hundreds of other sailors and Marines) on Iwo Jima's flight deck as the helicopter assault ship turned slowly north into the Hudson River.

It was Fleet Week, the city's annual Parade of Ships, and the beginning of a most poignant Memorial Day weekend.

Did they think to leave a space - a "missing man" space - for Tallon?

Here's hoping.

Tallon, 26, was assigned to Engine 10, right across Liberty Street from the World Trade Center; he was one of four from the firehouse to die Sept. 11.

The Marine Corps has this in common with the FDNY: They don't leave their dead behind, they just don't.

Bucking the odds, the Marines even brought the frozen bodies of their comrades down from the Chosin Reservoir, during that utterly heroic fighting retreat in Korea, 1950.

The FDNY, formally, will end the search for its own dead Thursday, as Ground Zero is officially given over to restoration and renewal.

And so life goes on.

Surely it does aboard Iwo Jima, which fairly sparkled in the sunlight last Wednesday.

Helicopters and Harrier jets fly off the warship's flight deck.

She carries hovercraft capable of putting main battle tanks, light armored and support vehicles across a contested shore with dispatch and precision.

In all, Iwo Jima can deliver some 1,500 heavily armed, highly disciplined and meticulously trained Marines to the fight.

Whether there will be an enemy present - and willing to resist - is an altogether open question these days.

The face of war has changed since Iwo Jima's namesake battle: Thousands died in one short week in that titanic March, 1945, struggle for a tiny but strategically vital volcanic island in the northwest Pacific.

Nowadays, America's enemies make war against civilians in no small measure because of ships like Iwo Jima - and the powerful fixed-wing-aircraft carriers and escort vessels that will accompany her into battle.

Conventional resistance to the United States, wholesale warfare, is seen to be futile.

Correct.

And so Operation Enduring Freedom is fought retail - one or two Afghanistan casualties at a time.

America is at war, and seems to be prevailing - and, happily, the casualties are limited. Try to buy a Memorial Day poppy on the streets of Manhattan. You can't.

And they don't sell poppies in memory of Sean Tallon and the 342 other firefighters who died on Sept. 11. Or for the cops. Or, even, the hundreds and hundreds of civilians.

But this doesn't mean that Tallon has been forgotten; nor will he be soon. Maj. Wally Powers and the men of the Second Battalion, 25th Regiment of the 4th Marine Division will see to that.

And should Powers and his Marines happen to cross an enemy coastline someday soon, and maybe come to grief, they won't be forgotten either.

The FDNY will see to that.

Lines cross oddly in this strange new post-Cold War world, don't they?

Say a prayer for fallen heroes this morning.

It's Memorial Day.

E-mail: mcmanus@nypost.com