Charles Widmore returns to the island in a submarine

Just off the island, we see a distant point-of-view shot of the beach camp reunion. Out of the water, a periscope appears. We then see to the point-of-view of the periscope which pans across the beach camp.

Inside the submarine, a man looking through the periscope says that there are people on the beach and asks if they should stop. We see none other than Charles Widmore tell him to proceed as planned.

So what exactly is Charles Widmore doing back at the island and which side in the coming war will he join?