'Birch' was the US geographical rather than operational codename for Christmas island within the Line islands group of the Pacific Ocean (1939/45).
Not to be confused with the Christmas island of the Indian Ocean, Christmas island of the Pacific Ocean, known to the US Navy as 'Gumdrop' and the US Army as 'Birch', is located near the equator to the south of the Hawaiian islands group and about 380 miles (610 km) to the south-east of Palmyra in the same islands group. Christmas island is one of the geologically oldest atolls known, and is of the raised coral type. It is about 32 miles (51.5 km) long and 17 miles (27.25 km) wide, with a total land area of 150 sq miles (388.5 km˛). The shallow lagoon, which opens to the west, is the Bay of Wrecks, and there are many small lakes scattered across the island. The island was populated by about 300 civilians when war broke out, 250 them local, 30 of them European and the rest Chinese. The island’s only product were copra and pearl shell from the lagoon.
The island had been discovered by the British on 24 December 1777 and annexed by the UK, but ownership was disputed with the USA in 1936, though neither party pressed the issue. The island had a small airstrip which constituted part of the 'air bridge' from the western coast of the USA and the eastern coasts of Australia and the Philippine islands group. The island received a small New Zealand garrison against the possibility of attack by German mercantile raiders in 1939, and the garrison had increased to 105 men and one 6-in (152.4-mm) coastal defence gun by the time war broke out in the Pacific.
The island was eventually garrisoned by about 2,000 US Army troops (single infantry, coast artillery and anti-aircraft artillery battalions) and the fighters of the 12th Pursuit Squadron from 10 February 1942 to help secure the sea lines of communications to Australia.