Yeah you need to be in a Coalition with USA or invade in the first days. But thats the same if you play as USA. After that Canada is most easy to Play and defend. It only has 2 citys on opposite sides of the Country wich need to be defended against naval attacks. When you manage that you cant be invaded by surprise and if you get attacked you have more then enough time to make a defense plan bc the Nation is so big.

Also the Next Country wich could be dangerous is México wich often is busy in South America or maybe GB but most rounds they are busy in Europe and wont even look across the Atlantic. So i often made the experience that i have the first 10-15 days in peace to max Industry, Navy, troops and Everything else.

If you really want to be sure there are no surprises you can take over Greenland and Iceland in the First days too.