Is it true that both sides thought that the war would be over quickly?

Yes.

They were proved wrong by the First Battle of Bull Run, after which there was no significant combat for six months.

There were very few officers on either side who guessed it would be a long war.

One of them was the too-elderlyUnion General-in-Chief, Winfield Scott,who proposed a long-term solution that was derided by the press as the 'Anaconda' (i.e. slow strangulation).

Also on the Union side, there was an unknown, half-madColonel, William T. Sherman, who made remarkably accurate forecasts to his friends in the South, explaining that they would enjoy some early victories, but that their lack of industrial manufacturing capability would increasingly swing the war against them.

On the Confederate side, it seems that only Robert E. Lee felt that it would be a long war. though presumably with a different outcome. Lee's view was highly emotional, fixated on the defence of his home-state of Virginia. Some say that hisblind spot aboutthe Western theatre cost the Confederates the war.