“When I have a problem, I call up Marco. He gets it solved,” Donald Trump boasts about his Secretary of State and National Security Advisor. This week, Marco Rubio is putting that reputation to the test. He is the key man on the two major international fronts the United States is currently engaged in, and with two very different roles: in the negotiations over the war in Ukraine, he took the reins to balance out what was initially a clearly pro-Russian proposal and try to push through a peace plan acceptable to Kyiv; in the standoff with Venezuela, he is...