The West. An alliance with China could be possible and supply lines to East Asia are critical. In addition there Pacific Fleet could blockade the Panama Canal and prevent any Chinese goods from getting to the other states in a reasonable amount of time. We also have a series of large mountain ranges that could slow down invasion heavily and plenty of oil, industry food and water. Plus the inevitable Capital in LA being really far away helps. The South is surrendering quickly and the Midwest becomes a grueling counterinsurgency campaign that claims tens of millions of lives but eventually ends in attrition from the Midwest due to the lack of trade and brutal Northern urban warfare.