When Tito was resisting Stalin/Soviet rule, according to the Wikipedia article on Tito:
An invasion of Yugoslavia was planned to be carried out in 1949 via the combined forces of neighbouring Soviet satellite states of Hungary, Romania, Bulgaria and Albania, followed by the subsequent removal of Tito's government.
But this invasion does not seem to have taken place. Why not? The article doesn't seem to explain.
The main Wikipedia article on the Tito-Stalin split is not quite as definitive about the alleged invasion plans, saying simply that:
Between 1948 and 1952, the Soviet Union encouraged its allies to rebuild their military forces-especially Hungary, which was to be the leading force in a possible war against Yugoslavia.
The 2005 paper War on Tito's Yugoslavia? The Hungarian Army in Early Cold War Soviet Strategy appears to cast doubt on the suggestion that the Soviet-bloc actually planned to invade Yugoslavia in 1949. It notes that:
"Béla Király, Commander-in-chief of the Infantry of the Hungarian army in 1949-1950, claims that Stalin had made a decision to resolve the conflict by military force"
"The attack had allegedly been planned from 1949 onwards, and the huge military build-up, exercises and the final war game on the basis of the “actual plan of attack” were concluded by the spring of 1951."
The reason that the plans were shelved was that, according to Király,
"… after the forceful answer of the United States to the North Korean attack on South Korea, the Soviet leadership thought better of their plan and quietly abandoned it after 1951."
Indeed, on 14 November 1951, President Harry S. Truman did ask Congress to approve military and economic aid to Yugoslavia.
However, that paper actually concludes, based primarily on declassified documents from both sides of the Iron Curtain, that
"the Hungarian military was not preparing for an invasion on Yugoslavia as Király claimed."
Although it does so with the important caveat that:
"… the archives responsible for defence policy in Moscow have partly or completely denied researchers access to their holdings for the period under discussion"
Why did the rest of the Eastern Bloc not invade Yugoslavia? Yugoslavia
It was a real possibility, especially after Marshal Tito blocked Stalin's supplies for the Greek Red Army in 1948, collapsing the Soviet sponsored revolution in Greece after WWII. But In 1953, Tito reached a mutual defense treaty with Turkey, which ultimately meant Yugoslavia was backed by NATO, while possessing a pretty significant armed forces of his own. Yugoslavia was after all one of only two countries in Europe, the Soviet Union being the other, which was able to push the Axis occupation out largely by themselves. (Soviet's and Bulgarian troops provided significant help to liberate their capital).
Tito was a master of playing the West and East against each other. His country requested and received aid in 1951 from the United States, during the the Marshal plan. At times during the cold war Yugoslavia recieved aid from both the West and Soviets.
Why didn't the Soviet Union invade Yugoslavia.
First, Yugoslavia wasn't a pushover. They had fought a long and bloody civil war which overlapped with WWII. They had given Hitler and Mussolini all they could handle and then some during WWII and Stalin wouldn't lightly involve himself in a war which he might not win.
Second, Yugoslavia, while not a member, was aligned with NATO. Yugoslavia formed the Balkin Pact with Greece and Turkey. Which pledged each to come to the other's defense in the case of Soviet invasion. Given Greece and Turkey were NATO countries this effectively placed Yugoslavia under NATO's security umbrella without the provocative move of actually formally admitting Communist Yugoslavia to the alliance.
The combination of both were enough of a deterrent to keep Stalin from invading. Although the Soviet Yugoslavian relationship definitely saw its strains.