Quote from trefoil:
Vietnam's a good example, and actually I thought I did get the point. The Brits sent more soldiers into the thirteen colonies than they ever had before to put down that rebellion, but wound up losing anyway.
Fighting on your own soil is a powerful thing, not to be underestimated. And trying to win a guerrilla war - the Revolutionary War, in large part, once the colonials were routed at the beginning, was a guerrilla war - is almost impossible.
You can also reference France trying to get a grip on Spain during the Napoleonic Wars. Wellington in support was a nice thing to have, perhaps, but the Spanish did a good job all by themselves of draining the French of a good part of the soldiers and treasure they could otherwise have used on their eastern front.