If you don't spend any money on your military except when you need it, it will take so long to get it back up to speed that you'll probably lose the war in the meantime.
And if you don't spend any money on renewable energy life for a large fraction of our species will become much harder.
And it's not like military spending is really necessary either. The US for example spent a lot of money in Afghanistan and Iraq even though the country wasn't under reasonable threat from those actors.
