WASHINGTON — America’s top foreign policy to-do’s in 2014 include preventing Iran from reaching the nuclear threshold, addressing the humanitarian disaster in Syria, containing an expansionist Russia, managing a rising China and reclaiming its own voice on human rights. Let’s take these one at a time. • Preventing a nuclear Iran: The global agreement over Iran’s nuclear program will last six months, after which U.S., Iranian, and other negotiators hope to fashion a permanent deal. But, Tehran insists it has the right to enrich uranium and it hasn’t committed to closing its heavy-water reactor at Arak, which could enable it develop nuclear weapons through the alternative route of producing weapons-grade plutonium.