I see snopes has claimed that this is probably a false Stalin quote:
America is like a healthy body and its resistance is threefold: its patriotism, its morality and its spiritual life. If we can undermine these three areas, America will collapse from within.
It’s quite unbeliveable Stalin’s ever officially said something like that.
It’d be the contrary to all the propaganda and the mindset of the USSR of that time. Stalin could say that “the communist revolution is inevitable in the US”, or that “the working class of the US will get rid of capitalist’s oppression”. The ‘collapse of the America’ was never the desire.