The mongol empire covered vast lands; the mongols themselves were known as

nomadic herdsmen with with excellent equestrian skills.
multiple generations of military people.
fierce and ruthless warriors.
explorers seeking additional agricultural land.

fierce and ruthless warriors.

Explanation:

The Mongol Empire was the second largest empire in history and the largest empire constituted by continuous territories. It was founded by Genghis Khan in the year 1206 and had its peak when it reached 24,000,000 km². It came to encompass a territory from the Korean peninsula to the Danube River, to host a population of more than 100 million inhabitants and to include some regions as rich and important as China, Mesopotamia, Persia, Eastern Europe, Russia, among others. The Mongol army was during the XII and XIII centuries the best in the world for its mobility and strategies, which made it feared among its contemporaries. Genghis Khan and other Mongolian military introduced several innovations that allowed his army to conquer vast territories, even being in numerical disadvantage during the battles they fought.

a big wall across the mexican border so no drugs, terrorist, or illegal immigrants can come through : )    

the answer to fill in the blank in this question would be the word resource, it is defined as the money, material or people (since human resource is a very valuable good) that a company or a person has at disposition (meaning they can use the resource as they see fit), to be able to complete a task or achieve a goal.