Yeah, the enemy pushes back. The AI calls in reinforcements to fight you every few minutes, so say you're fighting against Germans, first they usually call on a few squads of men and a bike, but as you capture points it moves up to a few squads and a Puma, then some StuGs, then some PzIVs, then Panthers, etc. The harder you have the difficulty, the more frequent the call-ins, and the harder the call-ins get... hardest difficulty I think the Germans call in KT's towards the end.
It's kinda shallow though, I prefered the SP in MoW1, and it's classicly bad voice acting.
Watch this if you want, German mission on Caen Outskirts. Guys playing on normal difficulty so I doubt they'll be any uber American tanks, but the mission can be fun once you get a Veteran Tiger.
http://www.youtube.com/watch?v=pu_aYKU3DGg