Five years ago, then as a candidate for U.S. president, Mr. Barack Obama made a triumphant appearance in Berlin. That speech drew 200,000 people and traced a direct line between the Obama candidacy and the power and imagery of U.S. President John F. Kennedy, who made a similar address 50 years earlier.

In an attempt to re-energize his agenda, Mr. Obama last week returned to Berlin and reasserted his vision of a world without nuclear weapons. That dream continues to animate the president — and remains controversial, as divisive as it is ambitious.

Just after taking office, Mr. Obama delivered a speech in Prague that outlined his vision of a world without nuclear weapons. In that address, he conceded that his goal would not be realized for many years, most likely not even within his lifetime, but he insisted — rightly — that such "distant" objectives must not be abandoned. The end of the Cold War may have ended the threat of nuclear annihilation, but the continuing existence of nuclear arsenals and the spread of their weapons pose a danger that is real and growing.

He followed that speech with strategic arms negotiations with Russia, striking a deal that cut the nuclear arsenals of the two former Cold War rivals to 1,550 strategic deployed warheads.

That was a notable accomplishment: The New START agreement signed by Mr. Obama and then Russian President Dmitry Medvedev was a two-thirds reduction from the level set by the original START treaty.

Critics noted that the agreement only addressed deployed warheads, leaving thousands more on the shelves available for eventual redeployment. Nor did it touch the thousands of tactical nuclear weapons that both countries retain.

Sadly the two countries' example was not followed by other nuclear weapon-possessing governments.

Among the countries recognized by the Nuclear Nonproliferation Treaty as "nuclear weapon states," China has most significantly expanded its arsenal, although the other two such governments (France and the United Kingdom) continue to modernize their arsenals.

India and Pakistan, which possess nuclear weapons but never signed the NPT, are expanding and modernizing their own stockpiles of nuclear weapons. Some experts reckon that India and Pakistan may now have more of these weapons than some of the recognized nuclear powers.

North Korea, which withdrew from the NPT in 2003, continues its nuclear programs, and Iran's nuclear ambitions remain shrouded. Plainly the nuclear disarmament movement has lost its momentum.

While Washington and Moscow may lead the way, other governments are not following their example. Undaunted, Mr. Obama has doubled down, returning last week to Berlin to resume his campaign. This time, he called for a one-third reduction in U.S. and Russian nuclear arsenals, bringing those stockpiles down to just 1,000 warheads.

Mindful of the omissions from New START, he also called for a cut in the number of deployed tactical nuclear warheads in Europe and promised to work with U.S allies in Europe and Moscow to find ways to do so.

Russia's response was cool. President Vladimir Putin has been an especially vocal critic of U.S. missile defense plans, warning that they could threaten his country's nuclear deterrent, a threat that becomes even more real as Russia reduces the number of nuclear weapons.

Russia is also worried about United States' conventional weapons that have great explosive power as well as extreme accuracy or "niche capabilities."

Finally, Moscow wants all countries' nuclear programs on the table. There is real concern in Moscow (and in Washington, to be honest) that China might "sprint to parity" if the two former superpowers reduce their arsenals to just 1,000.

For its part, China denies having any such intention, just as the U.S. denies that its antimissile systems would be effective against larger strategic arsenals (as in Russia or China) or that its "conventional strike option" has utility beyond the terrorists or other discrete targets.

Two competing truths hang over the nuclear debate. The first is that Russia is now more dependent on its nuclear weapons than it was during the Cold War. The once-mighty Soviet military machine has collapsed and Moscow cannot marshal the conventional forces that once so frightened Europe. Its nuclear weapons perform additional duties in defending the homeland and projecting Russia's power and status.

The second truth is that there is no standing still. If the established nuclear powers cling to their weapons and do not reduce their arsenals, other countries will follow them down the nuclear path.

The utility of such weapons — as either producers of status or security — means that other nations will no longer forego them. The established nuclear weapon states must delegitimize such weapons, and that message will only be sent and believed when those governments make real strides in dismantling their arsenals.

Japan is not just an onlooker in this process. A commitment to continuing nuclear disarmament is essential and Tokyo should be more vocal in its support for that goal.

At the same time, however, Japan benefits from the U.S. nuclear umbrella. Some of the most compelling objections to continuing reduction in U.S. nuclear weapons is the fear that allies will feel abandoned or insecure.

Japan, like other countries enjoying the U.S. nuclear deterrent, must make it clear to the U.S. (and the two countries' shared adversaries) that Japan supports such cuts, and that it has faith in other elements of the U.S. deterrent.

Japan does not need or demand nuclear weapons — either to possess its own or to depend on those of an ally.