Historians compare similarities and differences between societies primarily to

to collect evidence on historical events.

Explanation:

Collecting evidence on historical events is one of the main works of historians. As historians recognize, it is in most cases difficult to find get accurate information about historical events from one source.

For example, information about the famous Titanic ship that collapsed on April 15, 1912, were accurately gathered from eyewitnesses account and were then compared to find similarities and differences before arriving at a true story of what happened in history.

the crusades are one of the most significant events in the history of europe and the middle east.   they were a series of religious wars carried out by christian crusaders from europe during the timeframe of the middle ages.   beginning in 1095 ce, the crusades saw european knights and noblemen travel to the middle east in an attempt to capture the holy land away from muslim people that had controlled the region for the previous centuries.   the term crusade means ‘cross’.   therefore, the europeans that became crusaders viewed themselves as ‘taking up the cross’.   in fact, many of the crusaders wore crosses on their clothing and armor as they made their pilgrimage to the holy land.

explanation:

"a government in which power is divided between state and national levels". federalism is a system of government in which entities such and state and province, share power with the national government.