Who thought the civil war would be short?

Answer 1

Almost everybody on both sides.

There were two notable exceptions.

One was the Union General-in-Chief, Winfield Scott, who believed in blockading and starving out the Confederacy before sending in troops to destroy its weakened armies. He expected this to take at least a couple of years, and his plan was ridiculed as the 'Anaconda Plan' with its suggestion of slow strangulation. Also Scott was much too old and frail to be still in the job, however distinguished his record. He may have lacked the vigour to defend his plan.

The other was the unknown William T. Sherman, who was living in Louisiana and had made many friends in the South. To these friends, he wrote warning letters, explaining his view of the likely outcome, which turned out remarkably accurate. He said that the Confederates had certain miltary advantages that would bring them early victories, but their much smaller population (largely slaves) and lack of manufacturing industry would tell against them fatally in the second half.