This isn't fully formulated, but I suspect that Widmore is working with the MIB. In keeping with the theme that everyone cons John Locke because he's "amenable to coercion", let's say that everything Widmore said to Locke in Tunisia was a half truth. There is a war coming, but it's a war that Widmore wants. If Locke isn't on the island when the war comes then the wrong side will win, but that would be Jacob's side. Locke is special, but only because he was the candidate chosen by the MIB as the loophole.
I find it interesting that Widmore tries for 20 years to find the island, but Jacob dies and he's there in less than a week. I'm going to assume that the submarine figures heavily into the MIB's whole "leave the island and never look back" plan. Maybe Widmore made a deal with the MIB before being banished from the island. It puts Widmore's "torch the island" order in a little more perspective.
I also think that when the MIB told Ben that he wanted him to be "in charge of the island" that means that when the MIB leaves, then someone has to become the smoke monster. Dark vs Light, right? If there have to be candidates for Jacob's role, then that must mean that there have to be candidate's for Smokey's.