The Army and the Marine Corps make little appearances throughout the games from the very start: A good portion of the WC1 'reward' cutscenes have ground combat in them... with a special ops team blowing up the Kilrathi base at Brimstone, soldiers helping defend the Mopoks in Rostov and then interfering with the Sivar-Eshrad in SM2.
They're fairly absent in WC2, though we do see some heavily armored marines guarding Jazz on the O-Deck at the end of the game ("Betrayed the Confederation and nearly cost us the war, eh? Send him to play piano!").
If you kill your wingmen you'll see a couple of shipboard marines in WC3... and then they also appear in the Vagabond-punches-Dr.-Severin scene.
The marines get a huge amount of airplay (wordplay?) in the novels -- an amphibious (spacephibious?) invasion plays a major role in the second novel (End Run), and they appear throughout all the other books.