But the most effective method was to enlist a network of client states to take over the task, including Taiwan and South Korea, still under U.S.-backed dictatorships, as well as apartheid South Africa and the Argentine and Chilean dictatorships.
At the forefront was Israel, which became the major arms supplier to Guatemala. It provided instructors for the killers and participated in counterinsurgency operations.It is important to remember that the civil war in Guatemala took place in national, regional, and international contexts. The Guatemalan government was backed by these other governments during the Cold War but I am not sure that they needed the US to tell them to fear communism or how much the US need to tell them to support other anti-communist governments.
The Guatemalans also received support from these other governments because they would not abide by human rights conditions that the US was imposing in return for direct assistance, their preference for Israeli weapons, and their belief that the US approach to counterinsurgency was wrong as evidenced by its recent failure in Vietnam. The US still provided plenty of help but it is important to look at the entire picture.
For example, Taiwan and South Korea's communist neighbors obviously played a part in their interest in assisting anti-communist governments throughout the world. Latin American military officers trained in Taiwan. I'm not so sure whether they trained in South Korea but I wouldn't be surprised. Israel's Galil assault rifle became standard issue for the Guatemalan army.
The Argentines and Chileans were pretty gung ho about fighting communism even without the US. Operation Condor was more their own policy too that carried out attacks in the US, Europe and throughout South America. And Argentina was training the Contras in Nicaragua before they were Contras and before the US was even involved. Containing the Soviet Union and preventing its spread throughout the world was in lots of countries' interests. Even if Guatemalan and US representatives overplayed the connections of the Guatemalan guerrillas to the Soviet Union and hard core communism, their victory would have been perceived as a victory for the Soviet Union nonetheless.
On the other hand, the Guatemalan guerrillas received some international support but not too much. But the URNG and pre-URNG guerrilla groups did receive support from the Cubans, Mexicans, the Soviet Union and East Bloc countries, China, Vietnam, and Sandinista Nicaragua. There wasn't a lot of direct military support. It was more training, some money and weapons, and sanctuary. The URNG had safe houses throughout Mexico and each group had its own safe house in Managua as well as one for the entire organization (same as the FMLN).
While the URNG and the government of Guatemala received some international support, from what I understand so far, I think people on the left and the right tend to overestimate the actual level of international support to both sides. It's an effort on the right to de-legitimize the domestic source of discontent that allowed the guerrillas to survive for over three decades. And for those on the left, I think people overestimate US support because of their feelings towards Ronald Reagan and tendency to assume the worst of US involvement. There was also so many innocent people who were killed while the US supported the Guatemalan government that some of the details are irrelevant. I understand, and I was there myself as one point, but as an academic I'm not satisfied.