I can't seem to understand Japan's reasoning for declaring war on Britain after Pearl Harbor when at that point the US and Britain were not allies. Hitler did not seem to know that the Japanese were planning this attack (although he did welcome it after the fact) either, so it doesn't seem like there was any agreement between Japan and Germany before hand to declare war on both the United States and Britain before Pearl Harbor had taken place.
Eventually it seems that the U.S. and Britain were bound to be allies anyways after Germany declared war on the U.S. as well, but how could Japan have counted on that before Germany even knew of Pearl Harbor? It just seems to me like it would have been a wiser move to have not declared war on Britain until Germany and Japan had both agreed to declare war on their enemies. Not only that but it seems like Britain would have been less willing to dedicate resources to the Pacific theater if there was no immediate threat to them had Japan left them out if it, which from what I can tell would only serve to benefit Japan until they were able to defeat the Americans first.
Did Japan hope to grab British territory and resources as well?
Did they count on cooperation with Germany following the declaration of war on Britain?
On December 7 1941, Japan launched an attack on British, Dutch, and American holdings in the Pacific. This included Malaya (basically what is now called Malaysia), the Dutch East Indies (now Indonesia), Hong Kong, and Guam. They also bombed the US fleet at Pearl Harbor. So taking on the US and the British all at once was part of the Japanese strategy.
Furthermore, the US and the UK, while not formally "Allies", did have regular high-level contact and coordination. The US was already involved in a limited naval war with Germany. So it was also a forgone conclusion that the US and UK would be allies once the US entered the war.
There are a couple of misconceptions in this. The Japanese declared war on Britain essentially simultaneously with the Pearl Harbour attacks. The first acts of war against Britain by Japan were the landing of troops at Kota Bharu in Malaysia, and the movement of troops into Hong Kong. These both occurred on the 8th December, but this is in local time, ~18 hours ahead of the time in Hawaii. The landing at Kota Bharu occurred at midnight on the 8th in Malaysian time, or 6AM on the 7th in Hawaii.
As to why the Japanese would attack Britain, you have to look at the overall aim of the Japanese strategy. This was to secure the natural resources, especially oil, they needed to achieve economic independence. The majority of the oil available was in what is now Indonesia, at the time ruled by the Dutch. Oil was also present in the British possessions on the northwestern coast of Borneo. Rubber was also an important resource, present mainly in Malaya. To secure this, they would need to take the Dutch East Indies, and the sea lanes running to and from the islands. There were two major threats to these sea lanes. The major British naval base at Singapore, and the American naval and air forces in the Philippines. The Dutch were allied to the British as a result of the European war, so any attack on the Dutch East Indies would bring in the British. It was considered too risky to not attack the Philippines, as ships and planes based there could easily block troop convoys heading to the DEI. Such an attack would necessarily provoke an American response. The Attack on Pearl Harbor was an attempt to slow this response until the Japanese had been able to create a defensive perimeter in the Western and Southwestern Pacific. The aim of this perimeter was to slow and attrit the counter-attack, so that it could be defeated in a single decisive battle that would force the Americans to the negotiating table. Many of the islands considered necessary to form this perimeter, such as New Britain or the Gilbert and Ellice Islands, were owned by Britain or it's colonial dominions.
To sum up, the Japanese attacked Britain for four reasons. Firstly, to secure the oil fields of Brunei and the rubber plantations of Malaya. Secondly, they wished to capture the possessions of a British ally. Thirdly, they wished to secure and shorten their supply lines in South East Asia and Indonesia. Finally, they did so to create a defence against the American fleet.
Sources John Toland, Rising Sun: The Decline and Fall of the Japanese Empire 1936-1945, Pen and Sword Military, 2005
Peter Thompson, The Battle For Singapore, Piatkus, 2006