President Obama, speaking on Wednesday at Berlin's Brandenburg Gate, called on Russia to "move beyond Cold War nuclear postures" and join the United States in cutting its nuclear arsenal by a third.

"Our work is not yet done. For we are not only citizens of America or Germany, we are citizens of the world," Obama said, on the 50th anniversary of John F. Kennedy "Ich bin ein Berliner" speech. "We may not live in fear of nuclear annihilation — but as long as nuclear weapons exist, we are not truly safe."

The United States and Russia possess, by far, the most nuclear weapons in the world. Cutting down their stockpiles would be a big step towards global nuclear disarmament. Obama, however, faces some stiff barriers to implementing his plan.

1. Russia
During Monday's G8 summit in Northern Ireland, Obama and Russian President Vladimir Putin held a palpably uncomfortable joint press conference on the Syrian civil war that signaled a new low in relations between the two men.

That doesn't bode well for the chances of a new nuclear disarmament treaty. Russia already agreed to significant reductions when it signed the New START treaty in 2010, and has been reluctant to cull its arsenal further without concessions from the U.S. in other strategic disputes, including over the presence of a U.S. missile defense system in Eastern Europe. Tellingly, Russia has not commented on Obama's latest proposal.

As Obama noted, the Cold War is over. So why is Russia so wary of reducing its nuclear stockpile? Foreign Policy's Dimitri Trenin argued that since the fall of the Soviet Union, Moscow has relied on its nuclear capabilities to make up for its decaying army:

This deterrence operates at both strategic and tactical levels, making up for the huge gap in conventional capabilities between Russia and the leading military powers of the 21st century. Like the United States, Russia, of course, has inherited from the Cold War a nuclear arsenal which was absurdly large, thus allowing for massive reductions under the START and New START treaties — but now the smaller the numbers have become, the smaller the margin is for further reductions. [Foreign Policy]

If Moscow does agree to a new treaty, both the U.S. and Russia could reduce their respective arsenals to 1,000 active nuclear weapons, from the 1,550 both countries agreed to reach by 2018 under New START.

2. The GOP
Putin may seem like a formidable obstacle, but he's got nothing on Republicans in Congress — a group that is well practiced at obstructing the president's proposals. The GOP's opposition centers around the claim that the Obama administration has failed to follow through on certain aspects of the New START treaty.

"I remember the New START treaty debates where the administration made a number of promises in order to secure votes with regard to the modernization of our nuclear stockpile," Sen. John Cornyn (R-Texas) told The Daily Beast. "And to the best of my knowledge those promises have not been kept."

Republicans battled with Obama in 2010 over the New START treaty, and there is little reason to think they will back down this time around. When news leaked of Obama's plan before Wednesday's speech, Sen. Lindsey Graham (R-S.C.) warned of a "tremendous backlash" and asked, "What kind of signals are we sending? Our nuclear arsenal needs to be modernized."

3. The rise of other nuclear powers
The Cold War may indeed be over — but the collapse of a bipolar world has only led to a proliferation of nuclear weapons around the world. China, the Federation of American Scientists estimates, has around 240 nuclear weapons. India and Pakistan are believed to have about 100 each.

While Russia and the United States ramp down their nuclear programs, China is actually increasing its stockpile, adding 10 warheads to its arsenal in 2012.

The U.S. House has already passed a version of the National Defense Authorization Act that requires the White House to say it has "high confidence" in its assessment of China's nuclear capability before spending money on disarmament. It's a safe bet that Russia has similar concerns.