Why was the wording on war in Japan's postwar constitution so much stronger than in Germany's or Italy's?

In the case of Japan, the US took on the responsibility of defending Japan, while BRD (Bundesrepublik Deutschland) was encouraged and supported in a quest to rebuild the army (Bundeswehr) by the UK, US and (partly ?) France.

Germany (Prussia because this is about mentality) and Japan is different and also equal. Japan was ultra-nationalistic while Nazi-Germany less so while they were both militaristic (Prussian influence in Germany.)

Japan were ultra-nationalistic in that it's youngsters was encouraged and educated that it was a sacred duty to believe in the uniqueness of the emperor and to serve in the army and navy.

The reason appears to be the difference in the natures of the governments of the three countries.

In the case of the two European countries, it was believed that the governments had been "hijacked" by Hitler's Nazis and Mussolini's Fascists, respectively. The German version, in particular, appears to reflect this view: "Acts tending to and undertaken with intent to disturb the peaceful relations between nations, especially to prepare for a war of aggression, shall be unconstitutional. They shall be made a criminal offense." Basically, the prohibition was against behaving like Nazis, which would be "unconstitutional...a criminal offense."

In the case of Japan, while the country had similar leadership in Tojo, the centralization of power and authority in the Emperor suggested that the whole country behaved in a warlike way, not just the followers of Tojo. Therefore it was necessary that "the Japanese people forever renounce war as a sovereign right of the nation."

Emphasis (in italics) added by me.

USSR

While Western Allies (US in particular, since influence of UK and France was declining fast after WW2) certainly didn't want to make former Axis powers great again :) , few years after the war new reality appeared where USSR and communism in general became main opponents. Thus attitude towards each of these countries was shaped by this new reality . Let's examine case by case.

• Japan : Japan spent practically whole WW2 fighting against Western Allies, not USSR. Indeed, USSR invaded only in last days of the war, and Japanese certainly didn't want to fight them. There is one point of contention between Japan and USSR (Kuril Islands), but otherwise any Japanese military strengthening would have to be primarily naval and aerial, and only after that came land forces. At the time when Japanese constitution was adopted (1947) , communists haven't won in China and Korean War didn't start yet. Therefore, Americans didn't see any reason to recreate IJN that gave them such trouble just few years ago. Only after events in China and Korea Americans gradually realized they would need JMSDF, primarily as an anti-submarine force against Soviets and latter Chinese, while Americans would provide strike power with their carrier groups. Japanese air force and army were tasked primarily with defense of Japan (and surrounding airspace) in unlikely event of Soviet invasion. Idea to use these forces in hypothetical war in Korea and Taiwan was floated, but was mostly rejected because of historical reasons (Japanese occupation was resented in both places).

• West Germany : While Germans did fight against Western allies in WW2, bulk of their war was against USSR. After the war Germans were generally good at self-promoting themselves as expert anti-Soviet fighters and in blaming loss solely on Hitler. Americans mostly believed them, indeed former Wehrmacht officers were readmitted in Bundeswehr and some of them became high ranking officials in NATO. Of course, there were geostrategic reasons for all of that . Any Soviet invasion of Western Europe would have to pass trough Western Germany. Likewise, any NATO attack on Eastern Europe would have to start from Western Germany. Therefore, creating relatively strong West German army was in US interest. German constitution (Basic Law) came into effect in 1949, with Cold War already starting, and in vague terms considered East Germany as temporarily occupied territory but still part of Germany. It did prohibit offensive war, but defensive war was permitted. Of course, what constitutes defensive war is debatable. In any case, West Germany would not act unilaterally and would wait for US instructions.

• Italy: Italy is somewhat curious case and falls between Japan and Germany. During WW2 most of their fighting was against Western Allies, with some troops sent to aid Germans in Soviet Union. Generally, they were not regarded as tough opponent (with few exceptions), capitulated in 1943 and some of their forces switched sides and supported Allies. Italy was not on a main route of Soviet invasion or invasion into Soviet Union (Eastern Europe) but it had fairly strategic position in Mediterranean where Soviet subs and ships could appear, especially since some Arab countries (Egypt for example) for a time had very good relations with Soviets. It also should be noted that Italian communists had reasonable influence in the country, therefore Italian army could be used as a potential tool against their insurrection. Overall, Italians were allowed to have fairly strong navy, mostly as an anti-submarine force (similar to Japan) as this was viewed to be not too aggressive. Main strike power came again from US carriers. Italian army and air force were moderate, main difference with Japan was that they would certainly participate on some level in event of WW3 in Europe. As position of Italy was something of between compared to West Germany and Japan, so is the wording in their constitution. Italy rejects war as a means for settling international disputes (similar to Japan), but also forbids only aggressive war (like Germany) not war in general.