Stalin tried to kill Tito, that's what the threat was.

In history, Tito's letter will be remembered, through which he decided to defend himself and at the same time threaten the great Stalin.

In May 1948 there was a major rift between Tito and Stalin. Yugoslavia's Communist Party has dismissed the charges of Soviet communists against Yugoslav leadership policy. This was followed by a resolution of Informbiro and with continued political and economic pressure from the former Soviet bloc to Yugoslavia.

Tito then showed extraordinary courage and determination, not allowing himself to be involved in ideological disputes and insisting on state independence as a principle of relations between sovereign countries.
It is believed that more than 40,000 Stalinists (or those accused of Stalinism) have been subjected to depression between 1948 and the early 50th years.
Stalin is believed to have said that “just needs to move the little finger so Tito doesn't exist again”.
He has even personally organized several attempts at Tito's murder.
In the book “ (R. Medvedev, I. Medvedev, 2003) published a piece of Josip Broz Tito's letter to Stalin.
Stop sending people to kill me. We've already caught five men, one with a bomb, one with a gun. (...) If you don't stop with the commitment of the killers, I'll send one to Moscow and there won't be any need for another”, Tito wrote in the letter to Stalin.