The board was a map of the world. Each player started on one continent with a number of armies. Rolls of the die determined the outcomes of battles, introducing a certain element of chance, but good strategy usually triumphed.
One concept my younger brother never grasped was that one big army defending his borders was stronger than 3 or 4 small ones. But he liked spreading things out, so he'd have 4 armies each in Mexico, Central America, and Colombia rather than putting all 12 in Mexico.
Our older brother would come down through Texas with 6 armies and blow through like Santa Ana. Or whoever would have been blowing from the north.