KGB defector to Canada Yuri Bezmenov revealed the following:
The main emphasis of the KGB is not in the area of intelligence at all. Only about 15% of time, money, and manpower is spent on espionage as such. The other 85% is a slow process which we call either ideological subversion, active measures, or psychological warfare. What it basically means is: to change the perception of reality of every American that despite of the abundance of information no one is able to come to sensible conclusions in the interest of defending themselves, their families, their community, and their country.
It's a great brainwashing process which goes very slow and is divided into four basic stages. The first one being "demoralization". It takes from 15 to 20 years to demoralize a nation. Why that many years? Because this is the minimum number of years required to educate one generation of students in the country of your enemy exposed to the ideology of their enemy. In other words, Marxism-Leninism is being pumped into the soft heads of at least three generation of American students without being challenged or counterbalanced by the basic values of Americanism; American patriotism...
The result? The people who graduated in the 60's, dropouts or half-baked intellectuals, are now occupying the positions of power in the government, civil service, business, mass media, and educational systems. You are stuck with them. You can't get through to them. They are contaminated. They are programmed to think and react to certain stimuli in a certain pattern [alluding to Pavlov]. You cannot change their mind even if you expose them to authentic information. Even if you prove that white is white and black is black, you still can not change the basic perception and the logic of behavior. In other words for these people the process of demoralization is complete and irreversible. To rid society of these people you need another 15 or 20 years to educate a new generation of patriotically minded and common sense people who would be acting in favor and in the interests of United States society.
The East German Communist Rudi Dutschke said:
Revolution is not a short act when something happens once and then everything is different. Revolution is a long, complicated process, where an ordinary man must become different. The process goes along this way, which I have once named ‘The Long March through the Established Institutions’, in which institutions, through systematic Propaganda and direct actions, awareness is brought to further minorities in and outside the university, in schools, in trade schools, in engineer schools, also technical universities and finally in factories, where workers are currently worrying about their jobs. The process has begun, and that is a long process, which right now has been set on its course by us.
tl;dr: Communism isn't spread into free countries through warfare, or a sudden violent revolution. It is instead a long process lasting anywhere from a few years to a whole century during which communists infiltrate the Media, TV, journalism, education and all other institutions of a free country. Then, the people are slowly fed propaganda so that they will support Communism and eventually embrace it themselves.