Leon Trotsky, one of the original founders of the Bolshevik Revolution, was strongly opposed to the leadership and policies of Joseph Stalin. In response to Trotskys public criticism of him, Stalin banished him from the Soviet Union.
In 1924, Lenin died, and Joseph Stalin emerged as leader of the USSR. Against Stalin’s stated policies, Trotsky called for a continuing world revolution that would inevitably result in the dismantling of the Soviet state. He also criticized the new regime for suppressing democracy in the Communist Party and for failing to develop adequate economic planning. In response, Stalin and his supporters launched a propaganda counterattack against Trotsky. In 1925, he was removed from his post in the war commissariat. One year later, he was expelled from the Politburo and in 1927 from the Communist Party. In January 1928, Trotsky began his internal exile in Alma-Ata and the next January was expelled from the Soviet Union outright.
(Source: History Channel)
This seems odd. Stalin had no qualms about arbitrarily executing opponents (he even executed Trotsky's brother in law) Why would he exile one of his harshest critics instead of either killing him - or at the very least keeping him alive but imprisoned? What other factors were at play here ?