In late spring and early summer of 1942, there was an inter-service food fight in the American military: who first, Japan or Germany? It was supposedly settled by Roosevelt and Churchill at the ARCADIA conference in DC in Dec 1941: Germany would be the initial focus of the Allies because of the threat to Great Britain and the USSR, and Japan would be contained with what was left.
However, the Fall of Tobruk, the news of which was given to Churchill as he was meeting with President Roosevelt at the Second Washington Conference, greatly changed the focus in Europe. GEN Marshal, the US Army Chief of Staff, was pushing for an invasion of France for various reasons in 1942. With Churchill visibly shaken with the news of Tobruk, the two leaders began wargaming worst case scenarios, specifically a German-Japanese link up in India. In late June 1942, this wasn’t as farfetched as it sounds today: Japan was chasing British troops out of Burma and into India, India itself was on the brink of revolt, German formations were driving hard on the Caucuses, and Rommel seemed poised to seize the Suez and move on to Kuwait. Roosevelt offered Churchill all the help he wanted and purposely diverted convoys of material, specifically tanks, from the US to Egypt, at the expense of the troop buildup GEN Marshall wanted in England.
This diversion of resources made an invasion of France unfeasible in 1942 (if it ever was). It was obvious that the British, whose full support was absolutely necessary for a cross channel invasion of France, would not focus on France as long as the British Eighth Army was fighting in Egypt. As a consolation, General Marshall threw his support behind Operation Torch, the invasion of Vichy French North Africa in November. Roosevelt disapproved the plan, saying it wouldn’t help the Soviet Union, and told Marshal to keep working on a second front to assist Stalin.
But ADM Ernest King, the Chief of Staff of the US Navy, was concerned about the Japanese advances in the South Pacific and their threat to Australia. In particular he was worried about the recently discovered Operation FS, the proposed Japanese invasion Samoa, Fiji and New Caledonia, and the large airfield being built for it on the island of Guadalcanal. The airfield and Operation FS would cut Australia, New Zealand and New Guinea’s lifeline with the US. (Additionally, it was the Japanese who bombed Pearl Harbor, not the Germans…)
In an epic showdown with GEN Marshall and President Roosevelt, ADM King eventually persuaded the President to support his plan to stop the Japanese under the condition that the Navy and Marine Corps would do it themselves with no Army help outside of assistance from MacArthur’s troops fighting in New Guinea. The US Army would concentrate on an invasion of Europe and/or reinforcing the British in North Africa.
Immediately after the meeting, Admiral King cabled Admiral Nimitz, the Commander in Chief of the Pacific the news. And on 8 July 1942, Admiral Nimitz issued his orders to Marine Major General Paul Vandegrifft and the 1st Marine Division, then staged in New Zealand, to proceed with Operation Watchtower: the invasion of the islands of Tulagi and Guadalcanal in the Solomon Islands, set to begin in early August, 1942