Do not press an enemy at bay.
Bring war material with you from home, but forage on the enemy... use the conquered foe to augment one's own strength.
In the practical art of war, the best thing of all is to take the enemy's country whole and intact; to shatter and destroy it is not so good.
Weak leadership can wreck the soundest strategy
Weigh the situation, then move.
With regard to narrow passes, if you can occupy them first, let them be strongly garrisoned and await the advent of the enemy.