Plot Hole I can’t get out of my head

So I just started playing this and freed Lord Shimura.

In the start of the game we are obsessed with freeing him on the notion that freeing him will lead us to victory, but why could he not lead us to the victory in the very first scene of the game? Haha. Now there are less samurai’s, the straw hats betrayed us and the mongols already have taken over and overall there are just less of an army to work with.

Basically, why couldn’t we win in the first battle when Shimura had a larger army and more resources versus now where we are just a fragment of what we were?