Capitalism. Most of the other (daily, specific) dangers out there are dangerous because someone's making money off putting other people in danger. I'm including the military industrial complex, but also regular industries and the exploitation of vulnerable populations.
Necessary to prevent invasions by imperialists and capitalists who feel threatened by successful socialist models or who are looking to exploit other countries.
Imagine how much more they could have accomplished if they didn't have to fear the very real threat of foreign invasion. Remember, they were invaded by foreign powers shortly after the revolution in 1918.