It was bad.

To begin with, US-Japan relations were never great. They were both rising powers on the opposite ends of the Pacific, and so rivalry between the two was inevitable. There was always growing territorial tension between Japanese owned Taiwan and US owned Philippines, though it was no worse than many nations at the time. Japanese dominion on trade in Asia was seen as an obstacle to the US' own trading goals in China. Likewise, America's support of China was seen as a challenge to Japan's position as superpower in Asia.

The Japanese and American people also had a poor view of each other. Laws enacted within the US, such as the Californian Alien Law of 1913, were prime examples of this animosity. The Americans saw the Japanese immigrants as economic threats and unwanted guests in their land, and the Japanese, in turn, saw the Americans as hostile and xenophobic.

The divide between Japan and the West started with the Treaty of Versailles, where Japan felt ignored and drew the conclusion that the rest of the Allied Powers did not care for the Asian nation. This rift would only grow in the following years.

Ever since its conception in 1895, the Empire of Japan had Great Britain as its biggest supporter and, starting from 1902, ally. They relied on Britain both for public support and for logistical/strategic needs. In turn, Japan acted as a deterrence against Russia and joined Britain in wars. On 1921, the British demolished the Anglo-Japanese Alliance in order to gain the favor of the Americans. Knowing how unfavorable Japan was to America, Arthur Meighen, the Prime Minister of Canada, convinced the UK to drop the Alliance. Feeling betrayed, Japan took this as a sign that Britain had abandoned them in favor of the Americans. Distrust against Britain soared and vexation towards America grew.

The final nail on the coffin occurred in 1922, the year of the Washington Naval Treaty.

It was an international treaty between the US, UK and Japan detailing what the maximum tonnage of ships allowed was for each respective nation (as well as France and Italy).

During the Washington Naval Treaty, it was decided that a 5:5:3 ratio be imposed for respective three big navies. This meant that, at most, Japan was only allowed to have 3/5th of the fleet size of the US. Although the Western nations argued that this was plenty enough for Japan, the island nation saw things differently. The Japanese were infuriated at the results, interpreting the outcome as the US and UK seeing each other as equal, but seeing Japan as inferior. In their eyes, the entire conference was a power play by the US to assert dominance over Japan. They saw the US as purposely wanting to hamper the development of the Japanese Empire.

Prior to that point, the biggest theoretical enemy to Japan was Russia. Japan began amassing tanks in preparation for a ground war with Russia and practiced against it during wargames. Following the Washington Naval Treaty, however, the US began to replace Russia as the next most probable enemy of the Empire.