Heading west to Mexico from Petén is fairly straightforward and highly scenic in places, passing remote ruins and patches of dense rainforest. Though the Mexican state of Chiapas has been relatively calm for several years, tensions remain between government and Zapatista-aligned campesinos, and you can expect army security checks every hour or so as you get around. That said, travel is perfectly safe in the region, and the armed forces courteous and polite.
There are two popular routes. The first and most scenic involves crossing the Río Usumacinta into Chiapas at Frontera Corozal, from either Bethel, or a little upstream at La Técnica on the Guatemalan bank of the river. This trip enables you to pass the first-class ruins of Yaxchilán and Bonampak on the way. Alternatively, it’s possible to head northwest from Flores to El Naranjo by bus, and cross the border at El Ceibo into the Mexican state of Tabasco.