President Trump is prepared to double down on US sanctions for Russia to bring about an end to its war on Ukraine, retired Gen. Keith Kellogg, his special envoy to the conflict, exclusively told The Post this week — but he knows both Kyiv and Moscow will have to make concessions to end the “industrial-sized” killing in Europe’s largest country. Sanctions enforcement on Russia are “only about a three” on a scale of one to 10 on how painful the economic pressure can be, Kellogg said. The US sanctions themselves — such as those targeting Russia’s lucrative energy sector —...