Communist successes in 1947–1948 enabled them to move freely over much of mainland Greece, but with extensive reorganization, the deportation of rural populations and American material support, the National Army was slowly able to regain control over most of the countryside. In 1949, the insurgents suffered a major blow, as Yugoslavia closed its borders following the split between Marshal Josip Broz Tito with the Soviet Union. Finally, in August 1949, the National Army under Marshal Alexander Papagos launched an offensive that forced the remaining insurgents to surrender or flee across the northern border into the territory of Greece's northern Communist neighbors.