and now they can't employ the only way left to defeat entrenched Marxism
Fallacy. Nobody has ever overthrown "entrenched Marxism" by force of arms, since the entrenched "Marxists", as you call them, always have tanks, APCs, bombers and better arms. With the possible exception of Afghanistan and other countries where a strong religious element could fire up the resistance, the best that has been done to undermine a one-party socialist regime is to force the ruling party to agree to a peace treaty and elections supervised by the UN or some other international body. This was the case in Nicaragua and Cambodia. Afghanistan was practically a Soviet invasion, their equivalent of the Vietnam war, so might not even count as an "entrenched" regime but a regime that was bound to fall the minute the armed foreign support was withdrawn.
The collapse of one-party states in Europe at the beginning of the Windows era was due to a number of factors, partly economic, partly political (notably the attempts by Gorbachev to reform Soviet socialism), and partly pop-cultural. Hollywood, Coca-cola, and McDonalds can claim more credit for the fall of Iron Curtain than guns or CIA subversive schemes, unless, of course, Hollywood and McDonalds were CIA schemes.
By the way, not all Marxists are communists (i.e. supporters of a Communist party). There are many paths to the tower. While most Marxists probably believe in the necessity of violent social upheaval, some consider this as simply the natural consequence of captialism's so-called internal contradictions and adopt a more cautious attitude, basically just waiting for the state to collapse rather than taking up arms to bring it down.