Would it be accurate to say secession was about slavery( in some states), the war was about the federal govt imposing its will on the states?
That secession was about slavery would be correct.
The second part is kind of true, but in a hypocritical way of the part of the South. They South was perfectly fine with the Federal Government imposing its will on states when the South saw it to their benefit. They pushed through and supported the Fugitive Slave Act, a Federal law that required states where slavery was illegal to actively participate in the capture and return of fugitive slaves to their masters. The Southern states openly supported the Dred Scott case, that ruled that a Slave owner could take his slaves into a Free State and that Free State had to legally recognize the masters right to his slaves even thought slavery was supposed to be illegal in that state.
