Roger Cohen repeats  a very strange assumption:
But, some say, a firm response will end Russian cooperation on vital issues like Iran. Not so: Russia has its own interest in stopping nuclear proliferation [bold mine-DL], and even the Cold War did not preclude cooperation in some areas.
It’s possible that Russia won’t subvert negotiations with Iran in retaliation for sanctions over Ukraine, but it is still impressive how ready so many people in the West are to believe that inflicting punishment on Moscow won’t produce undesirable results on other issues. We should certainly hope that tensions over Ukraine don’t poison cooperation with Russia in other important areas, but it seems to me that this is the sort of thing that the administration is telling people to make punitive measures seem less risky than they are. Bear in mind that the administration officials that assure us  that Russia has its own reasons for cooperating on Iran also think that recent Russian actions in Ukraine aren’t in Russia’s interest. In other words, Moscow has already acted in ways that the White House believes to be inconsistent with Russian interests, and that suggests that the administration may not be the best guides to understanding what the Russian government currently perceives its interests to be. It is very convenient for advocates of sanctions to argue that a punitive approach won’t jeopardize U.S. goals elsewhere, but that doesn’t make it true.
The assumption is that Russia has as much reason to prevent a nuclear-armed Iran as anyone else, but we should also remember that Moscow has been much less alarmed by Iran’s nuclear program than Western governments are. Russia may not want Iran to have nuclear weapons, but it seems much less worried that Iran is likely to acquire them. The other argument is that Russia doesn’t want to sabotage diplomacy with Iran because that would make U.S. military action more likely, but it’s not so obvious that it would greatly harm Russian interests if the U.S. and Iran couldn’t resolve the nuclear issue peacefully. Russia benefits in some ways from ongoing U.S.-Iranian hostility, and it is not harmed if the U.S. ends up waging yet another war in the Near East. The administration takes for granted that diplomacy with Iran won’t be undermined by sanctioning Russia, but this seems to be a case of believing that the U.S. doesn’t have to make a trade-off between its desire to punish Russia for annexing Crimea and its goal of reaching a comprehensive agreement with Iran. The latter is presumably more important to the administration, so we should assume that Russia will extract a price for its continued cooperation.