American cities are very spread out, and most heavy industrial sites are located in suburbs or in the middle of nowhere. Destroying parts of Manhattan or Capitol Hill might have some psychological effect, but does nothing to dampen the ability of the U.S. in continue fighting a protracted war. If you want to fight a nuclear war with the U.S., you would need megaton-type warheads to maximize destruction and casualties. A few fission bombs will not hamper America's ability to wage war continuously. On the contrary, China and other Asian countries are characterized by everything (especially strategic heavy industries and research facilities) locating in the middle of major cities (Shanghai, Wuhan, Dalian, Hefei, etc.), so dozens of W88s and W87s would be enough to bring China to its knees. I am not saying the U.S. is completely invulnerable, but you need a lot more (and much more powerful) warheads to inflict the same amount of damage to the U.S. as the U.S. could do with just a few warheads to China. However, this could change overnight if China were to enter a war economy and relocate most heavy industries to mountainous areas (say suburbs of Sichuan, Shaanxi, Yunnan, Guizhou, Ningxia, Western Hunan, Western Hubei, etc.), especially all the underground tunnels China already has.