U.N. Secretary General Ban Ki-moon met with Russian President Vladimir Putin on Friday, saying it is important not to "lose momentum" in the effort to convene a peace conference on Syria. Ban was only the latest in a string of foreign dignitaries who have come to Russia, seeking Putin's blessing for such a conference, expected to be held in early June. There's a lot at stake. Russia has been a long-time supporter of Syrian President Bashar al-Assad and continues to supply weapons to his regime. U.S. officials have said lately that those weapons include advanced missile systems for attacking ships and airplanes. If Assad already has such weapons, they could pose a real threat to international efforts to impose a no-fly zone, to deliver supplies to the rebels, or to maintain a maritime embargo.