Advice from Lama Zopa Rinpoche
War - Inner and Outer
While having lunch at a coffee shop during His Holiness the Dalai Lama's teachings in New York City, Rinpoche met a nun in the Kagyu tradition who said that Rinpoche was the first lama she had met, years ago.  She told Rinpoche that she had done two 3 year retreats, but was not a good student. Rinpoche went on to say:

"I think she was being polite. So I told her you should always remember that retreat means making war on delusions.
You have to always remember this: this is inner war, but this is a one-time war. Once you defeat delusion by ceasing the cause, the seed of delusion, by actualizing the remedy, which is the path, it doesn't come back again. Once the seed is removed the delusion never comes back, so this is just a one-time war. Outside external wars are endless, because you have to fight again and again.

In Shantideva's Bodhisattvacharyava tara, Shantideva mentions that the outside enemy never ends, however many you are able to kill. Shantideva used the example that if the whole earth is covered by thorns so that you want to cover the whole earth with leather, there is not enough leather to do that.  However if you have leather under your foot, it's as if the whole earth is covered by leather. Shantideva said the meaning is that once the inner enemy, delusions, are destroyed, it's as if all outside enemies are destroyed."