Martial Law is not restricted to a state of war: e.g. 1970 in Canada. Also court martials: those happen also when a country isn't at war.
Well yes, and you don't need to be married for marital law to apply. The question of martial being an adjective form of war is quite unrelated to the question of under what circumstances exactly it applies. But check for translations of martial law; most of them will be exactly 'law of war'. Of course nifty politicians will impose it whenever it suits them, not only when it is linguistically applicable.
In modern usage, the word military is used as an adjective, but it's clearly a noun in its inception.
Well, whatever is the inception of a word anyways? Musical seems to be pretty noun-y, but of course it is an adjective form in Latin. Military stems from a Latin adjective; it is borrowed from French militaire, and I can't seem to find out whether that also was a noun at the moment it was borrowed into English, but surely began as an adjective. It is nowadays used very often as a noun, and English doesn't distinguish adjectives and nouns in any clear manner, so we really have no idea how to find out whether military is primarily an adjective or noun these days, but evidence seems to point to it beginning its life as an adjective.
Almost hypocritically, I will point out that the actual adjective of military -- militant, has fallen out of common usage as such, being replaced, in general by "martial". Yes, it refers to Mars, but what you've forgetting is that all soldiers are "men of war"; They're all in martial professions. The act of committing war is linguistically inseparable from the act of being a soldier.
Militant actually derives form the Latin present participle militans (with stem militant-), (imagine a present participle like English gerund -ing), from Latin
milito 'to be (or act as) a soldier, to make war'. So in Latin you have
militaris - of or pertaining to the military
martialis - of or pertaining to Mars, by extension to warfare
militans - being a soldier, acting as a soldier, making war
Of course, all their meanings are extremely close to each other, and will often be interchangeable, so there's no real way of clearly demarcating one from the other, especially in English, where their meanings have changed (militant meaning something like warmongering, or aggressive). Also it's never correct to argue for systematic relations between words (such as A is the adjective form of B) from etymology; went is not related to go etymologically; still it's a form of it. So in the end we are reduced to speakers' intuitions. To me - as a foreigner -
military sounds like it relates to the body of the military whereas
martial sounds like it relates to the state, or the business, of war; that's why I thought it's incorrect to call
martial the adjective of
military. However, if the natives agree on this actually being the case, there is little left to argue about.
