During the second phase, Japan took advantage of the weakening British hold on the region to invade Siam, seeing the country as an obstacle on the route south to British-held Malaya and its vital oil supplies, and northwest to Burma. On 8 December 1941, after several hours of minimal fighting between Siamese and Japanese troops, Thailand acceded to Japanese demands for access. Later that month Phibun signed a mutual offensive-defensive alliance pact with Japan giving the Japanese full access to Thai railways, roads, airfields, naval bases, warehouses, communications systems, and barracks. With Japanese support, Thailand annexed those former possessions in northern Malaya it had been unable to acquire under the 1909 treaty, and conducted a campaign against its former allies in the Shan states of Burma along its northern frontier.