Defence and Defense:

I've been looking at some 1800s American legal sources, and I noticed that many still used "defence" well into the second half of the century. At some point, though, "defense" became the universal American term. Do any of you know why the "-ce" shifted to an "-se"? Did the "-our" / "-or" shift and the dropping of "-me" on "programme" and the like happen for the same reason? If the latter shift was part of a spelling simplification movement, why the "-ce" / "-se" change, which isn't really a simplification?