They are our allies now, but out of curiosity I Just wondered if there were any proscriptions from the WWII surrender agreements they signed, that would legally or otherwise contractually limit their ability to increase their miltary forces and hardware to the levels they desired or saw fit, without input from anyone else.
I’m inclined to doubt it… Japan has rather famously refused to develop nuclear weapons thus far, but that’s a choice, not something they’re prevented from doing by law.
Article 9 of the Japanese constitution states:
However, the current interpretation is that while Japan renounces war “as means of settling international disuptes,” war in self defence is acceptable and therefore it is constitutional to maintain resources to this end. Even though Japan’s military budget is the second highest in the world, this is justified as a “Self Defence Force.” Also, the Japanese people are theoretically allowed to change the constitution without consulting any foreign or international authorities. (I say “theoretically” because it hasn’t happened so far, and the conditions are rather strict: 2/3 votes in both houses of parliament plus 50% popular vote.)
Japan’s “ban” on nuclear weapons is strictly a voluntary declaration and is not legally binding in any way. The administration can simply choose to overturn it if they want. And some politicians are starting to talk about doing just that.
Japan also has a sizeable defence industry. They are not allowed to export weapons, but IIRC this is also a self-imposed ban and there are some special cases which are allowed. Many weapons used by the Self Defence Force are produced in Japan including US-designed fighter aircraft produced under license. They also have a fair amount of American weaponry including four Aegis cruisers. And just last month the Japanese government launched their own spy satellite. The current one is only as good as commercial imaging satellites, but better ones are being developed.
The short version is that there are a lot of people who would be very nervous and quite a bit more aggressive if either one started to seriously re-arm.
wrt Germany: The Two-plus-Four Treaty of 12 September 1990 limits German armed forces size to 370,000 (for comparison: Cold War strength for West Germany alone was 480,000). Current head count is about 315,000, reduction to 280,000 planned. Germany also is a party to the various conventional-forces limitation traties, nuclear non-proliferation treaty etc. etc.