Japan was (and remains) explicitly barred from re-establishing an open military under Article Nine of the 1947 Constitution, which states
"(1) Aspiring sincerely to an international peace based on justice and order, the Japanese people forever renounce war as a sovereign right of the nation and the threat or use of force as means of settling international disputes. (2) To accomplish the aim of the preceding paragraph, land, sea, and air forces, as well as other war potential, will never be maintained. The right of belligerency of the state will not be recognized."

While the wording of this clause has allowed the establishment of a de-facto military in the Japanese Self-Defence Forces, their whole structure and indeed very name is deliberately constructed around the idea that it is not a military.

In the German case, there was no specific constitutional prohibition against a military within the Grundgesetz (the Basic Law), although there was something similar to the first subsection of Article Nine in Article 26;
"Activities tending and undertaken with the intent to disturb peaceful relations between nations, especially to prepare for aggressive war, are unconstitutional. They shall be made a punishable offense."

However, when the Bundeswehr was established in 1955 after the collapse of the attempt at a joint European defence force, there were an array of specific safeguards put in place around it. The Bundeswehr is explicitly for defence and other specific purposes outlined in the Grundgesetz only. Any uses beyond this (such as peacekeeping missions) are subject through Constitutional Court decision to an explicit mandate from the Bundestag concerning its maximum size and rules of engagement.

The US occupied Japan after World War 2, and wrote Japan's new constitution for them. The Japanese had some input into the writing of the constitution, but they were living under US military control so were limited. So article 9 of the Japanese constitution says that Japan renounces war and cannot have a standing army. Your father is right. But it's more complicated. Japan has a Self Defense Force. That's basically a military in all but name. There is a large US military presence on Japan to defend them too. It would also be possible for Japan to repeal article 9 and have a military openly. But most Japanese people would oppose this and that's why it hasn't happened. Yes US imposed the rule on Japan of no military. But Japan could easily change that if they wanted to. The vast majority don't want to. But for the minority who want remilitarization, the Self-Defense Forces are ready and waiting to be turned into regular military.