Another fun exercise in numbers, would a platoon of US Marines be able to hold off a Roman Legion in combat.
The short answer is almost certainly, yes.
While a Cesarean Era Roman Legion has a greater than 100x personnel advantage the ground combat tech that a Marine rifle platoon has would give them an incredible series of advantages, and that’s before the Marines history of getting into fights with numerically superior forces and winning, even with near peer tech bases.
A Marine platoon with logistical support would be able to break any combat force of that size given the technology disparity and methods of fighting. The Marines can put lethal controlling fires over roughly a square kilometer without moving, depending on terrain. Romans would have to advance into that lethal area, taking casualties the entire time.
The Romans are a close contact force. Primarily armed with melee weapons and ranged weapons designed for engaging formations of enemies relatively similar to their own. Their ranged capabilities, up to and including siege weapons would be of limited use as their heaviest weapons couldn’t effectively target individual dispersed Marines 10-15 meters apart, in cover. Their maximum effective ranges would put them under fire from the Marines and the bigger weapons would make much larger and slower targets for the marksmen.
Roman legionaries would need to close through the 500-600 meters of deadly terrain, one that would be chosen or moved to by the Marines either prior or during the contact with the large and very obvious legion. 43 Marines could easily hide from, harass, and inflict non-reciprocal casualties on a legion. Once 20% of the Romans had been taken down, dead or wounded, that battle would be in the hands of the Marines. Across historical precedent no large units have sustained 20% casualties and remained combat effective.
The limiting factor, the one there would have to be enough Romans to outlast, would be ammunition. In contact the Marines would have an almost entirely one way casualty flow until their ability to shoot ran out. Marines would withdraw as part of their fire and maneuver planning long before they ran dry on ammo. The smaller, more mobile, and harder to find force could then evade effectively against the larger legion, even harassing them with gunfire and stealing supplies. Especially a force equipped with suppressors for the M27’s and M38’s.
Only through an astronomical expenditure in manpower that would be psychologically untenable could a legion overcome the Marines. So in short, they can’t.