The Marines [and the authors of the idea] should have checked with Hannibal Barca. He killed something like 80,000 Romans in one afternoon [one of three major battles he won], and still lost the war.
And although the Romans ceased colonization after Teutoberg Forest, they destroyed most of Western Germany over the next three years.
The Marines would suffer more casualties from the local water, than they would the entire Roman Army, in the field.
The marines would have to use their superior firepower to enlist recruits, maybe inspire major defections from the Roman government and military. They need to leverage the awe into some kind of capitulation by the Romans.
But if they just start out somewhere and commence fighting, they will run out of ammo and fuel.
Germanicus being removed from the rhine by tiberius may have altered the long-term outcome there. Rome, at the time, seemed to view the loss of varus legion as a setback which they quickly set to making right, hunting down those involved, recovering eagles, etc.
Yep, relentlessness was their course of victory for centuries.