WITH THE FIRST MARINE DIVISION, in Iraq, March 27 — Marine and other allied units pressing toward Baghdad are coming under nearly constant harassment and ambush by small bands of irregular Iraqi fighters and remnants of army units they bypassed in their rush, and officers fear the resistance will only stiffen as they get nearer the capital. "We've been contested every inch, every mile on the way up," Col. Ben Saylor, the First Marine Division's chief of staff, said today. Even as he spoke, a separate Marine unit, Task Force Tarawa, was in the fifth day of a pitched battle...