From back of the case:
Embrace technology, govern wisely, and build an invincible military force for war on land and at sea. Only then will Nobunaga's Ambition be realized and a nation united under one sword.
Quote from user manual pg. 14:
You assume the role of a Sengoku Era daimyo and take control of a clan (also known as a force). By building facilities and learning new technologies, you can increase the strength of your force, and through diplomacy and fighting battles, can increase the size of your territory. The ultimate objective of the game is to unite the entire land under one daimyo.