Burma was separated from British India in 1937 as a Crown Colony with a constitutional government. The colony was important for sending supplies to China via the Burma Road, which was completed in 1938. Nevertheless, it was left lightly defended at the outbreak of World War II: the British considered it a backwater and unlikely target of attack. Japan began its conquest of Burma with small raids in December 1941, launching a full invasion the following January. Japan held most of the country by April and ceded the Shan states to its ally Thailand.