Here are two good pre-battle messages, although the second was printed and distributed, not actually spoken to the troops in person:
“The time is now near at hand which must probably determine whether Americans are to be freemen or slaves; whether they are to have any property they can call their own; whether their houses and farms are to be pillaged and destroyed, and themselves consigned to a state of wretchedness from which no human efforts will deliver them. The fate of unborn millions will now depend, under God, on the courage and conduct of this army. Our cruel and unrelenting enemy leaves us only the choice of brave resistance, or the most abject submission. We have, therefore, to resolve to conquer or die.”- George Washington, Address to the Continental Army before the Battle of Long Island (August 27, 1776)
"You are ordered abroad as a soldier of the King to help our French comrades against the invasion of a common enemy. You have to perform a task which will need your courage, your energy, your patience. Remember that the honour of the British Army depends on your individual conduct. It will be your duty not only to set an example of discipline and perfect steadiness under fire but also to maintain the most friendly relations with those whom you are helping in this struggle.
"The operations in which you are engaged will, for the most part, take place in a friendly country, and you can do your own country no better service than in showing yourself in France and Belgium in the true character of a British soldier. Be invariably courteous, considerate and kind. Never do anything likely to injure or destroy property, and always look upon looting as a disgraceful act. You are sure to meet a welcome and to be trusted; your conduct must justify that welcome and that trust.
"Your duty cannot be done unless your health is sound. So keep constantly on your guard against any excesses. In this new experience you may find temptations both in wine and women. You must entirely resist both temptations, and, while treating all women with perfect courtesy, you should avoid any intimacy.
“Do your duty bravely. Fear God. Honour the King.” - Lord Kitchener, Field-Marshal, Guidance to British Troops (August 4, 1914)