Before you can bring in a reserve unit to the battle, one of your units that is already present must either be withdrawn from the battlefield (CTRL + W), be completely destroyed, or have routed off the battlefield. My advice would be to withdraw a unit that is severely depleted or has routed once before.

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Quote You can gaze upon the lords, but looking at the shogun will make you blind; and the emperor cannot be seen at all.[/QUOTE] -Old Japanese saying