A Japanese-American war in the middle of the First World War was, as you said, implausible by 1917, but it wasn’t impossible. Zimmermann didn't pull the Japan clause out of nowhere: German diplomatic strategy had long entertained the idea of convincing Japan to switch sides.

German Misconceptions

The idea of a Japan-German alliance was a popular idea in German foreign policy, and German diplomats had been hard at work trying to broker a separate peace with Japan since 1915. Diplomat Paul von Hintz, who had previously been assigned to Mexico, had already reasoned that brokering a Japan-Mexico alliance could divert significant resources away from the American-Entente trade. Negotiations taking place in April and May of 1915 saw Japan considering peace in exchange for German colonies in Asia, with a more subtle understanding that doing so would free Japan to either switch sides or wage war on the United States. However, Japanese diplomats had been playing the Germans for fools. Japan already occupied the German territories in China and the Pacific, and were only looking for the guarantee that the occupation would transfer into ownership after the war. The Japanese foreign office therefore leaked the German promises to the British government, playing the two sides against each other. In this, the Japanese were wildly successful, obtaining guarantees of obtaining Tsingtao after the end of the war from the Entente and effectively killing the initial impetus behind a Japanese-German alliance.

As the German war situation deteriorated, however, they tried again in 1917. These efforts actually spearheaded by Zimmermann himself, and he instructed diplomats to resume talks with the Japanese Ambassador to Sweden. Once again, these efforts were used by the Japanese in order to pressure the Entente to be more favorable to Japanese war goals, rather than actually entertaining the idea of switching sides. What was more plausible, if only slightly, was the idea of a Japanese-Mexican alliance, which Japan itself had subtly promoted in order to pressure the United States into accepting Japanese interests in Asia. To this end, Japanese diplomats maintained close ties to Mexico, including an arms deal in 1917 that explicitly went against an agreement in which Japan would only sell arms to the Entente. It’s safe to say that Zimmermann took inspiration from this when writing his infamous telegram, but in the end misunderstood Japanese diplomacy for being more literal than it really was. Once again, this all played to Japan’s advantage, with the two-fold punch of the negotiations with Germany and overtures to Mexico proving sufficient enough for both Britain and the United States to acknowledge Japanese interests in China, with each respectively signing the Anglo-Japanese Agreement of February 1917 and Lansing-Ishii Agreement.

But in conclusion: Did Japan seriously consider going to war with America in 1917? No.

Sources for this section: Source Records of the Great War, Vol. V, ed. Charles F. Horne, National Alumni 1923

Ikle, Frank W. "Japanese-German Peace Negotiations during World War I." The American Historical Review 71, no. 1 (1965): 62-76. doi:10.2307/1863036.

Japanese-American War

But did people think such a war could happen? Outside of the immediate context of World War 1, it seemed a lot more likely.

In the West Coast, Japanese paranoia was everywhere: by 1917, pro-German propaganda had already been playing the Yellow Peril card on America in a somewhat misguided attempt to convince Americans that war with Japan was imminent. A particular favorite of mine is from the Fatherland newspaper, which ran a 10 chapter story depicting a fictional American war against an Anglo-Russo-Japanese-Mexican alliance. Relevant passage:

On October 12, 1920, all California law prostrate before an alien foe; before those two alien foes who long had threatened, one the despised foe of Mexico, the other the feared foe of Japan ; and before foes who had not threatened, but struck, treacherously, suddenly, during the very speeches of friendship - England, Canada and Russia.

It’s a pretty fun read, and ends with Germany coming in to save the day. An ironic plot development, considering that the Zimmermann telegram was intended to start the very same kind of war. You can read the story here .

The Fatherland paper here was playing on the pre-existing deep seated anti-Japanese sentiment on the West Coast at the time, particularly in California, where Japanese immigration had set the state aflame with controversy. The San Francisco Chronicle helped kickstart this with a multi-editioned series on the perils of Japanese immigration in 1905, which greatly bolstered the anti-Japanese movement with the subsequent formation of the anti-Japanese League. The state senate subsequently passed a number of anti-Japanese laws, such as restricting them from owning property and segregating them in schools. The unjust treatment of their nationals in America ruffled feathers in Japan, and though it didn’t do much to drive Japan to war, it certainly did a lot to convince the California public that such an event was likely: Senator Phelan of California was a staunch anti-Japanese crusader, and his fiery anti-Japanese rhetoric at times played on trumped up charges of Japanese colonization efforts in Northern Mexico and Southern California, citing a Japanese-American conspiracy to get Japanese into California in violation of immigration agreements between Japan and America. Numerous editorials and opinion pieces in California speculated on the possibility of a war with Japan, and many welcomed the idea. However, the main argument for a Japanese-American war had less to do with immigration in America, and more about who would dominate Asia:

The rest of the nation wasn’t as worried about immigration, and California saw its anti-Japanese legislation blocked by the federal government in the name of international diplomacy, with Theodore Rosevelt blocking the law on school segregation, the later Gentlemen’s Agreement on immigration in 1908, and Wilson blocking the revised Alien Land Laws in 1919.

What was more important on an international scale was the conflict between the American Open Door Policy in China and Japan’s desire to colonize her neighbors. This was an issue that the Japanese and American governments actually came close to clashing over, and arguably was the main reasoning behind World War 2. A lot has been said on this, so I’ll skip over the details, but I’ll elaborate on the Japanese perspective, as it really did almost lead to a very different World War 1.

In 1914 Japan, the question of whether to support the Central Powers or the Entente was split between the warring factions of the Elder Statesman Yamagata and Foreign Secretary Kato Takaki. Yamagata had argued that a German victory would be preferable as it would shatter the dominance of the Anglo-sphere over the world, particularly in Asia, which would essentially give Japan free rein in China. Kato aligned himself with Britain and the Entente due to a personal belief in parliamentarian systems and acknowledgement of the benefits that being a junior partner under Britain could bring. The Kato faction would ride a rising movement of increased parliamentary influence and support for civilian government to get the Japanese government to honor their treaties and join the Entente, but Fredrick R. Dickson argues in his book that it was a much closer call than we really give it credit for.

Sources for this section:

Viereck, George Sylvester. “War of 1920” The Fatherland v. 3 no. 2. New York : International Monthly Inc. 8 August 1915. Accessed via Digital Library at Villanova University

San Francisco Chronicle, University of California Newspaper digital archives - I looked into this for a project, so I don’t have the exact paper issue numbers, but can try to find them if needed.

War and National Reinvention: Japan in the Great War, 1914-1919 by Frederick R. Dickinson