Why would a 1939 Japanese southward offensive struggle?

How much territory could Japan seize in a war beginning in early 1939?

Its prospects would be constrained by distance, logistics, the Philippines, and easier Allied reinforcement; a southward advance would be far harder than in 1941.

Major Japanese landings in early 1939 would be much harder than those of December 1941 because Japan lacked the later launching points in French Indochina. The Philippines blocks the route toward Borneo and the rest of Southeast Asia. British reinforcements could land into friendly ports with food and support already available, while an invading force would have to carry and sustain everything it needed. Japan had four high-mobility divisions based largely in the southern home islands, and perhaps two could be sent against the Philippines. Their long transport and supply route would still make success doubtful. Soviet pressure on Manchuria is also plausible because border fighting was already under way, the USSR supported Chiang Kai-shek, and memories of the Russo-Japanese War supplied another motive.