Early in the war, why was lincoln's greatest fear that the war be perceived as one against slavery rather than one in defense of the union

Because the fight against slavery was not Lincoln's original intention. The Civil War was more to unite the Confederacy back with the Union rather than further separate the two. It can be also seen as since the South favored slavery, the being against it could possibly split America at the time further apart

Because he proposed that all the treaties should be made through the league of nations.

From mexican american war , us gained new mexico.