The Fighting 69½th (1941)
A picnic; no humans in sight. Both the red and black ant colonies spot it at the same time. Their scouts get into a fight over the first olive, and of course you know, this means war. The war features such gags as limburger stink bombs, commando raids on the cake, and a bucket brigade building a sandwich. But the human returns, gathering up everything but one cake. The generals get into another fight trying to divide it.