Hapoel Hatzair
In 1919, Kaplan was a member of the Ukrainian delegation to the Versailles Peace Conference. The following year he immigrated to Mandatory Palestine, and was involved in merging "Youth of Zion" with Hapoel Hatzair to form Hitachdut, and following his participation in the Conference of the Zionist Federation in London, was elected to the Zionist Executive Committee. A short while later he was sent to Berlin to run Hitachdut's world office.