If you wait until you are actually being invaded to start preparing for it, you're gonna have a bad time. It takes about 6-9 months to properly train a new soldier, so you need to start planning for a cadre of soldiers well in advance of any actual invasion.
Honestly, even waiting until the first rumblings of a possible invasion seems to start preparing contingency plans is too late, because once the rumbling start, people will complain that anything that visibly attempts to resist it is "too provocative."
Honestly, even waiting until the first rumblings of a possible invasion seems to start preparing contingency plans is too late, because once the rumbling start, people will complain that anything that visibly attempts to resist it is "too provocative."