To many in the north, they were fighting to

keep the union intact
allow slavery to expand
keep secession legal

The North was primarily fighting to abolish slavery while keeping the Union in tact. The North was upset when the South officially left the colonies, since it made them look weaker to other countries, as they themselves were on the brink of civil war. They did not want secession to be legal for this reason, but at the same time they did not agree with the South's view on slavery.

the crusaders are identified in the painting;

c. by the red crosses on their backs