The USA is particularly strong from that side. Most of its navy patrols those waters, many islands on the Pacific are USA territories all the way to Japan which itself acts as an arm of the USA Military since the forced demilitarization of Japan in WWII. Japan and other islands basically form a wall around the eastern mainland that would be difficult to cross undetected, and when they did reach the shore they might get flanked from Hawaii, Panama, or Alaska.

There is a reason Stalin chose to ally with and provide nuclear weapons to Cuba, and approach the USA via the gulf of mexico and Atlantic.