Complex sentence: He had urgent reason to claim she was dead at the moment of his marriage in Villers-Cotterêts, France, in 1792: he would have been required to consult her opinion on the marital union.
Split into simpler sentences: He had urgent reason to claim she was dead at the moment of his marriage in Villers-Cotterêts, France, in 1792. He would have been required to consult her opinion on the marital union.