Russia is no longer worried about the United States. They are too busy watching Obama destroy it to think about having to do the job themselves.
I think you are correct. The greatest danger to the USA comes from within; the list is long and well known.
Invasion from Russia or China is impossible. The USA mounted the best ever defense - it made itself unappealing. What kind of a foreign power will want to inherit 100+ million people who only know how to cash welfare checks? Who would need 6 million aggressive and armed "disadvantaged teens" (per the latest PC dictionary?) That crowd is larger than the combined armies of the USA and Russia.
The only conqueror who could tackle the problem is Genghiz Khan. He would just say "I want the land; I don't need the population." Today such a method would be considered inhumane.