I believe very strongly that slavery could have been abolished without Lincoln's War, but that would present two problems. 1) Lincoln wanted to strengthen the federal government and essentially abolish States' Rights. He needed a war. 2) Southeners had more to fear than a slave insurrection. Just as Northerners resisted free blacks from moving to their states (see the first Black Codes), Southerners did not want free black people to assimilate into their society.
When the European countries emancipated their slaves in the colonies, they did not forsee them coming to live in British, French, Dutch, or Spanish society.