Britain only has so many troops to send at any given time, and the US is very methodical. You have years before they are of real concern. Much of what happens in this game is about patience and timing, its not all about rushing. Its about playing at a proper pace to take advantage of each element. For example if your Japan its easy to conquer Nationalist China, but if you do annex it as opposed to firing seize the coast, the USA embargoes you (they are your best trade part) and you kick their threat up faster (which is bad for you and your allies). Everything is a trade off.