Appealing to Hungarians with promises of regaining the land lost to neighboring countries within a week of his rise to power, Kun declared war upon Czechoslovakia as Hungarian forces invaded Upper Hungary on May 20, capturing southern territories within weeks. In the face of advancing Hungarian troops, the Allies began to put pressure on the Hungarian government and, within three weeks with Kun's assurances of Russian support failing to materialize, Hungary was forced to withdraw from the just proclaimed Slovak Soviet Republic after given an ultimatum from France, together with a guarantee that Romanian forces would retreat from Tiszántúl.