> civil war where the heroes are universally agreed to be the side fighting for more powers for the federal government.

The Union wasn't fighting for more powers for the federal government, just for the integrity of the existing federal government and it's powers.

The war resulted in some expansion of federal power, but that wasn't what motivated the Union.

Sure, that's a reasonable way to look at it, and the only way consistent with having the victors write the history books. In the same way that the Commerce Clause always allowed the federal government to regulate growing wheat for personal consumption, the Constitution already allowed the federal government to forbid slavery - it just took a civil war or a supreme court hearing to make sure everyone agreed on that. Under that view, the 13th Amendment was legally redundant, but was passed just to leave absolutely no room for doubt.

> the Constitution already allowed the federal government to forbid slavery

The Union wasn't (particularly the slave states in the Union weren't) fighting to abolish slavery, the Union wasn't fighting to preserve the Union, a goal to which the popular (in most of the North) cause of abolition had been deliberately subordinated in the elections prior to the war.

OTOH, by seceding, the rebel states also lost their leverage on preserving slavery within the Union, so the whole rebellion backfired. Abolition probably would have happened eventually without the war, but it wouldn't have been right away.