Why does Napoleon decide that trade with humans is a necessary evil? How does he explain that to the animals? What is their reaction? How does he ultimately win them over?

Napoleon insists trading with humans is necessary for them to acquire materials needed for their farm.All the animals were reminded of the earlier commandments: "Never to have any dealings with human beings, never to engage in trade, never to make use of money." Many animals protested this new trade agreement but Napoleon quickly silenced all opposition. He decreed that Mr. Whymper would be their middle man. Squealer quickly gets to work putting his serious spin on what the animals knew.He claims that the earlier resolutions never really happened.

"He assured them that the resolution against engaging in trade and using money had never been passed, or even suggested. It was pure imagination, probably traceable in the beginning to lies circulated by Snowball."