The Marines and the Marines on Sipan

The marines travelled lightly advanced quickly, were willing to suck up more casualties in the hope that, by ending a campaign sooner, the overall casualties would be lessened. The army was much more deliberate and preferred to wait for things like artillery support. On sipan, it's caves. And japanese soldiers will hide in these caves and if the caves are not dealt with, they will come out after the americans move past them - this makes matters more complicated and tragic.