At the time of this telegram German forces were defeated. The American
and British forces are scaling down their armies in Europe and transferring
forces to fight against Japan.
As Churchill wrote, Soviet forces controlled most of Eastern Europe. A number
of disagreements arose about what was to happen in the Eastern European countries
once Germany was defeated. Vienna was one of many cities which the Americans
and Britain did not want to occupy but the USSR did.
In many Eastern European countries Communists had played an important
role in resisting the Nazis (although many non- Communists also resisted
the Nazis as well). Many communists used their support from the war and
support from the Red army to take control of the government of countries
where the USSR drove German forces out. This is what Churchill meant by
the Communist technique.
The USA, Britain and USSR reached a number of agreements at a conference
at Yalta in February 1945. You can read more about this in the section on