On The Scene: A Formidable Foe

(CBS)  CBS News Correspondent John Roberts is traveling with the U.S. Marines making for Baghdad and filed this report from the town of an-Nasariyah.

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The shattered American armor on the streets of an-Nasariyah says it all. The Marines are dealing with an enemy that is both far more determined -- and far more capable of disrupting their schedule than anyone thought.

And it is an enemy that has quickly adapted to find weak points in America's military might.

For all of its firepower the U.S. Marine Corps can't move very far or very fast without the vast logistics trains behind it to back it up to rearm and re-supply. The Iraqis are employing a strategy in this conflict, trying to tie up those logistics trains preventing the Marines from moving north.

At every point along the roads leading north the Marines have been caught up in fierce firefights. The word from troops on the ground is that rather than regular army troops -- or even militia, they believe they are meeting elements of the Republican Guard that have traveled south to blunt the American offensive.

That may be true -- or not -- but the very mention of it is an indication the opposing force is a tough one.

The Marines moved today to end the bottleneck caused by the weekend battle at an Nasiriyah -- punching thousands of troops and hundreds of vehicles forward. But many of those troops ended up stopping to guard the road against possible attack.

And the Marine unit we are with, which fought a ferocious battle last night, was again heavily engaged tonight.

Pentagon officials today insisted today the war timetable is on track, but the Marines are already at least 36 hours behind their preferred schedule. They are now faced with the prospect of having to protect every inch of a lengthening supply line raising the question, does the U.S. have enough "boots on the ground" to occupy a hostile country as large as Iraq?