Editor’s Note: Seth Moulton, a Democrat, is a Marine Corps veteran who represents Massachusetts’ 6th Congressional District. The views expressed in this commentary are his own. Read more CNN Opinion.

CNN — Twenty years ago this month, I was a young Marine lieutenant marching down a California street in an Iraq War victory parade. We had defeated Saddam Hussein and the Iraqi Army, and we thought it was all over. But of course, it was not. Twenty years later, American troops are still in Iraq. 

Israel is about to start an invasion of Gaza with the same mission we had in those early days in Iraq: defeat the enemy, or in Netanyahu’s words, “crush and eliminate” Hamas.

But the lesson America quickly learned in Iraq and Afghanistan was that you have to have a plan for the day after. Outside the southern Iraqi city of Najaf during the invasion, then Colonel David Petraeus famously asked, “Tell me how this ends….”

The next year, I found myself in that same city Petraeus had conquered, trying to win it back. It was my second of what would become four tours in Iraq, facing far worse combat than we saw during the invasion.

By that time, the insurgency was entrenched, much like Hamas is in Gaza today, and we had to fight, block by block, to root it out.

One day in the summer of 2004 reminds me of how difficult this was and why you always have to consider what comes next.

My company was called to rescue an Army Special Forces detachment from a school in which they took serious casualties. After we helped evacuate their wounded and dead, my Marines and I assaulted the building with a platoon of Recon Marines in support.

But with insurgents throwing grenades from the roof and rolling them down the stairs, we took casualties ourselves, and I made the difficult decision to withdraw my platoon — and call in an air strike to level the building.

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