QUOTE
That's no excuse. A threat could have come from outside the galaxy - or, as we saw, from within. Frankly, if the Republic had so little regard for its own protection then it deserved to fall.
Well, considering how astronomically far galaxies are from each other, a threat outside the galaxy probably seems a little out there... Literally!
And a threat from within... well, the people of the Republic probably thought some local police forces could handle anything like that. Or maybe they had a draft system, or something like that. Or maybe they used to have an army, but after hundreds (maybe even thousands) of years of peace, it just sort of fell out of service.
As an analogy, imagine you are a guy. You used to get into a lot of fights, but for the last 20 years or so, you've lived a peaceful life and your not in the best of shape anymore. It doesnt really seem necessary. Then, suddenly, some other guy picks a fight with you! So, of course, you arent very good at defending yourself, and get beat up. Thats what it was like for the Republic!