I feel like I'm taking crazy pills here. It seems like every decade I've been alive, a major power declares their intent to arm/train "freedom fighters" in a strategically important nation or proxy war. Right now the US is the primary offender, but this seems to be modus operandi of many major powers throughout modern history.
My question is - are there examples where this actually worked?? Here I'd define success as "achieving a strategic goal, and the armed dissidents didn't become/lead to a whole different strategic problem in a decade."
Well, isn't the american independence war a perfect example? The 13 colonies were clearly the underdogs at the beginning of the war and foreign support (from the french or spanish, even the prussians through the sending of officers to train the continental army) played a big role in the final victory against the british.
Materiel and training for WW II resistance fighters, particularly in France, seems to have been worthwhile. Of course that was in the context of an active war.
The US had a string of semi-successes in the Cold War. In 1954 Eisenhower's CIA was able to pull off a successful Coup d'Etat in Guatemala using a rogue portion of the Guatemalan military forces. Similarly, the 1961 US/Belgian assassination of Patrice Lumumba in the Congo led to seizure of power by more US friendly forces (though this really only lasted for three decades and now the Congo has been home to the world's longest Civil War). The US/British coup against Mohammed Mossadeq installed the shah for 2 and a half decades though it ultimately led to the Ayatollah. The US sponsored Pinochet coup in Chile also was successful for several decades.
The longest running one though was the fascist sponsorship of the Franco regime which led to four decades of fascist dictatorship in Spain. So yeah, these are successful regularly but they will cause eventual blowback that society will have to reckon with.