At the end of the Great War, only three naval powers remained in the world; The United States, The United Kingdom, and the Empire of Japan. Historically (circa 1900) Japanese naval doctrine was to keep a force of what could be classified as capital ships equal to 70% of the United States. Their theory was that if any war broke out between the IJN and USN, then two separate engagements would occur, in which the US Navy's Pacific Fleet, and later Atlantic Fleet would be crushed decisively.

The Japanese delegation to the Washington Naval Conference was of two separate mindsets. The former was of the 70% doctrine, while the second was well aware of American production capability compared to the Japanese Empire. For comparison, the average GDP of Japan in 1920 was only 18% of the United States, and the IJN held only 55% as many capital ships as the USN. The WNT came with a prohibition of all fortifications built in the Pacific, perhaps seen as a windfall by the Japanese, already feeling threatened by British fortifications in Burma, Hong Kong, and Oceania, as well as a large US presence in the Philippines.

The Japanese were very wary of the WNT, but ultimately accepted in favor of building a cruiser rich naval doctrine, shown in the battles off the Solomon Island chain, as well as smaller engagements around the Philippines in early 1942. The WNT remained a controversial topic in the IJN, as well as the Japanese Government until open hostilities in 1941.

SOURCES Stephen Howarth's "The Fighting Ships of the Rising Sun"

Text of the WNT http://www.ibiblio.org/pha/pre-war/1922/nav_lim.html

EDIT: Formatting

Everything fellow redditors have mentioned is correct, I only have to add that you have to take the WNT in the context of US' 1910s plans for Navy development which the Japanese were well aware of via naval attaches in Washington:

The Wilson administration's plan for Naval rearmament from 1916 and its expansion in 1919 included a ludicrous amount of over 50 first line battleships, in order to surpass Royal Navy's number of 42 front line battleships. This was an insane number that Japan's Eight-eight plan for construction of 8 BBs and 8 battlecruisers, which was already seen as a huge industrial endavour simply couldn't match. This plan however, provoked the political aspects of the Royal Navy even more than it had ones in the IJN.

After Wilson left the office, the succedding Harding administration realized the plan was ludicrous and set out to a diplomatic solution - however, the genie was left out of the bottle and the threat of USN focusing its industral effort on creating a huge invasion fleet was hanging over Japanese heads.

Kato Tomosaburo, Navy Minister at the time, while in the first being a stark proponnent of the eight-eight plan and exponential naval armament, eventually realized an armament race would be a situation in which Japan would be the first one to drop out. However, Kato Tomosaburo's viewpoint was not shared by the majority of the IJN staff; most unexperienced young officers saw the arms race as a display of national prestige and the signing of the WNT was seen as something paramount to treason. The loudest proponent of this was Kato Kanji, a young officer who argued that US's industrial capacity is the exact reason why Japan shouldn't sign the WNT - since in the case of a war, USN will just be able to build the extra ships in matters of months, whereas IJN would be stuck with its feeble fleet which couldn't be expanded as quickly.

At the signing of the treaty, both Kanji and Tomosaburo were present in the Japanese delegation - whereas Tomosaburo accepted all US requests, Kanji threw a huge tantrum and the delegation almost came to the point of rupture; eventually Tomosaburo managed to contain the delegation, mostly due to the help of higher political factors from back home in Japan that reminded Kanji of his place as Tomosaburo's subordinate . However, this didn't mean Kanji stayed quiet afterwards and due to this incident, the IJN quickly "split" into the anti and pro Treaty factions, which eventually calmed down, but resurged again in 1930 as the London Naval Treaty appeared as an extension of the WNT.

Sources:

Evans&Peattie: Kaigun, p 191-198

Buckley: The United states and the Washington conference