Short answers, pulled from the story.
The Washington Naval Treaty, also known as the Five-Power Treaty, was signed on the 6th of February 1922 by the British Empire, the United States, Japan, France, and Italy. It limited the construction of battleships, battlecruisers, and aircraft carriers and established tonnage ratios among the signatories to prevent a post-World War I naval arms race.
The treaty set a capital ship tonnage ratio of approximately 5:5:3:1.75:1.75 for Britain, the United States, Japan, Italy, and France respectively. The British Empire and the United States each received 525,000 tons of capital ships, Japan 315,000 tons, and France and Italy 175,000 tons each.
Japan gave formal notice of termination on the 29th of December 1934, and the treaty's provisions expired at the end of 1936. Opposition within the Imperial Japanese Navy, particularly from Fleet Faction officers who viewed the 5:5:3 ratio as a strategic handicap, combined with broader ultranationalist pressure drove the decision. Japan had also renounced the Second London Naval Treaty in 1936.
Hughes opened the first plenary session on the 21st of November 1921 with the declaration "The way to disarm is to disarm" and immediately proposed a ten-year construction holiday on capital ships, scrapping of existing and planned vessels, and the 5:5:3 tonnage ratio. His dramatic opening received strong public endorsement and likely accelerated agreement on his proposals.
Yamamoto, who had served at the Japanese embassy in Washington and observed American industrial capacity firsthand, argued that Japan should remain within the treaty. After it was signed he stated, "The ratio works very well for Japan; it is a treaty to restrict the other parties," and separately that anyone who had seen the auto factories in Detroit and the oil fields in Texas would know Japan lacked the power for a naval race with America.
The American Cypher Bureau, commanded by Herbert Yardley, intercepted and deciphered the communications of multiple delegations throughout the conference. Japanese cables were read thoroughly, giving American negotiators advance knowledge of the absolute minimum concession Japan would accept before walking away from the negotiations.