Why did the union want to win the war?

Answer 1

Before his election, Abraham Lincoln made it clear that his goal, for the good of the entire nation was to preserve the Union. Once the first shots of the war were to attack Union military forts, Lincoln was able to galvanize the the Northern States to commit to this goal. Secondly, Lincoln knew that if the Confederacy was beaten that slavery within the United States would be abolished.

Keeping the Union intact would also prevent Mexico from even attempting to win back some of the territory it lost in the Mexican War. Keeping the Union intact would also prevent any European country from subverting Mexico to be a puppet nation for any European power.