You really don’t need to go after the USA, but if you must, this is sometimes fun…
X= whenever opportunity knocks
On turn X, land 6-8 inf in Alaska. The USA will amass forces to come up to Alaska, but don’t worry, Alaska is not your objective… On turn X+1, move all naval units to Sea of Japan for reloading. On that same turn, lots of USA infantry will probably move into W. Canada, and that’s exactly what you want. On turn X+2 move your entire fleet to Hawaii, and take it with AT LEAST 10 infantry. This move is called “Check.” Now the USA has many land units in W. Canada, but has to worry about the invasion of California. Which direction should the units go? They could take Alaska, but it leaves W. Canada open and makes the American player heavily rely on whatever infantry he places in CA at the end of his turn. He could move everything to CA, but Canada is open again, and the Jap threat in Hawaii remains… either way, you’ve just disrupted about 4-6 turns of US movements to Europe… Germany can breathe now.