I don't know of any army in the world that would be armed and equipped the way you describe. 2500 is closer to brigade or regimental strength then battalion. Where are their supporting arms and what are their capabilites? How did they get to the good ole USA? Airdrop? Amphibious landing? Smuggled in trucks across the Mexican or Canadian border?
Are there other bridges or fords between the Moronian forces and your town?
The easy answer is to blow the bridge at let the river keep them out.