Build fast units, motorised infantry, light tanks in the first few days to invade Canada/Mexico with.

Build a wall between USA and Mexico - that'll slow them down. Invade Canada and take all the maple syrup - feed it to the Mexicans to keep em happy. Build up forces in Panama and then invade through the Central America taking Capitols and boost the morale of the Canadian provinces you captured, then take Mexico.

There is really no best strategy chiseled in stone because every game is different depending on who their leaders are. If you have far more experienced players controlling Mexico and/or Canada than yourself, it would be wise to form a North American coalition so you don't get bogged down in a war with any of them. I've played the USA in historical WW2 four or five times and won them all with solo victories, but only because the three of us didn't fight and I didn't need to produce units to survive. All of my resources and dollars were then put into upgrading resource acquiring buildings and raising my technology. If Europeans are fighting each other, you are now laughing. Those involved are naturally forced to buy as many units as they think necessary to keep their offensive going or to defend against what ever force assails them. This often leads to their minimal factory upgrades and advances in technology, that you don't have to worry about. The USA must have the greatest patience to do this for about a week and resist the temptation to buy units first. Of coarse once you have spent everything needed to raising your tech and economy, you can then build what ever units you can still afford, with what you have left over, or if you are really patient, save some so you can have enough to pay for the ever increasing costs of higher tech and resource producing factories. In all the games I've played as the USA, Canada has always quit early while Mexico did so only once, but if that happens don't be tempted to invade them unless you can take them out with minimum casualties to yourself and do it quickly. I mean in less than 24 hours is best for a few reasons and two days at the absolute latest, of course taking the capital last so you don't have to waste troops garrisoning provinces that might revolt if you get them after you take Ottawa. Another strategy is to stay friendly with A.I. and let them live and build up their own defenses to protect your normal border, while you sit back getting stronger and look for opportunities across the Atlantic or Pacific. I once saw Spain take almost half their original starting army and land unopposed in New York while the Yank army and navy were too far away in Canada to do anything about it. The yank was just about to deal the final blow to the Canucks but turned around and fled back to deal with this unexpected and ultra rare event. The Spanish timing on taking Boston, Philly, New York and Washington couldn't of been better as I learned later. An hour after they were captured they produced units the Americans had started long ago that could of at least hung on to Washington until the rest of their army arrived. The Canuck followed the retreating Yanks who failed to dislodge the Spanish invaders. The two of them had lost about 90% of their army so the Canuck attacked them both and eventually took over the entire USA less than a week later. I never saw anything like that happen again except when I play Japan and land on the west coast sometimes. Ah man,someone is knocking on my door,so I gotta go.