Ornithopter said:
I don't really understand why the Republic had essentially no army available to go against them trade federation.
because of plot force and it sounds poetic - a millennia of peace.
They had an army until about 1000 BBY when the last Jedi/Sith war occurred and because the Sith were "defeated" they no longer needed an army, and the Jedi assumed the role as peacekeepers.
while there were skirmishes between planets and systems, they were generally resolved pretty quickly. It was not until the Trade Separatists showed up that it became something that impacted a larger part of the galaxy, requiring an army.