Republican presidents have long led the crucial fight to protect the United States against nuclear dangers. That is why Presidents Richard Nixon, Ronald Reagan and George H.W. Bush negotiated the SALT I, START I and START II agreements. It is why President George W. Bush negotiated the Moscow Treaty. All four recognized that reducing the number of nuclear arms in an open, verifiable manner would reduce the risk of nuclear catastrophe and increase the stability of America's relationship with the Soviet Union and, later, the Russian Federation. The world is safer today because of the decades-long effort to reduce its supply of nuclear weapons.

As a result, we urge the Senate to ratify the New START treaty signed by President Obama and Russian President Dmitry Medvedev. It is a modest and appropriate continuation of the START I treaty that expired almost a year ago. It reduces the number of nuclear weapons that each side deploys while enabling the United States to maintain a strong nuclear deterrent and preserving the flexibility to deploy those forces as we see fit. Along with our obligation to protect the homeland, the United States has responsibilities to allies around the world. The commander of our nuclear forces has testified that the 1,550 warheads allowed under this treaty are sufficient for all our missions - and seven former nuclear commanders agree. The defense secretary, the chairman of the Joint Chiefs of Staff and the head of the Missile Defense Agency - all originally appointed by a Republican president - argue that New START is essential for our national defense.

We do not make a recommendation about the exact timing of a Senate ratification vote. That is a matter for the administration and Senate leaders. The most important thing is to have bipartisan support for the treaty, as previous nuclear arms treaties did.

Although each of us had initial questions about New START, administration officials have provided reasonable answers. We believe there are compelling reasons Republicans should support ratification.

First, the agreement emphasizes verification, providing a valuable window into Russia's nuclear arsenal. Since the original START expired last December, Russia has not been required to provide notifications about changes in its strategic nuclear arsenal, and the United States has been unable to conduct on-site inspections. Each day, America's understanding of Russia's arsenal has been degraded, and resources have been diverted from national security tasks to try to fill the gaps. Our military planners increasingly lack the best possible insight into Russia's activity with its strategic nuclear arsenal, making it more difficult to carry out their nuclear deterrent mission.

Second, New START preserves our ability to deploy effective missile defenses. The testimonies of our military commanders and civilian leaders make clear that the treaty does not limit U.S. missile defense plans. Although the treaty prohibits the conversion of existing launchers for intercontinental and submarine-based ballistic missiles, our military leaders say they do not want to do that because it is more expensive and less effective than building new ones for defense purposes.

Finally, the Obama administration has agreed to provide for modernization of the infrastructure essential to maintaining our nuclear arsenal. Funding these efforts has become part of the negotiations in the ratification process. The administration has put forth a 10-year plan to spend $84 billion on the Energy Department's nuclear weapons complex. Much of the credit for getting the administration to add $14 billion to the originally proposed $70 billion for modernization goes to Sen. Jon Kyl, the Arizona Republican who has been vigilant in this effort. Implementing this modernization program in a timely fashion would be important in ensuring that our nuclear arsenal is maintained appropriately over the next decade and beyond.

Although the United States needs a strong and reliable nuclear force, the chief nuclear danger today comes not from Russia but from rogue states such as Iran and North Korea and the potential for nuclear material to fall into the hands of terrorists. Given those pressing dangers, some question why an arms control treaty with Russia matters. It matters because it is in both parties' interest that there be transparency and stability in their strategic nuclear relationship. It also matters because Russia's cooperation will be needed if we are to make progress in rolling back the Iranian and North Korean programs. Russian help will be needed to continue our work to secure "loose nukes" in Russia and elsewhere. And Russian assistance is needed to improve the situation in Afghanistan, a breeding ground for international terrorism.

Obviously, the United States does not sign arms control agreements just to make friends. Any treaty must be considered on its merits. But we have here an agreement that is clearly in our national interest, and we should consider the ramifications of not ratifying it.

Whenever New START is brought up for debate, we encourage all senators to focus on national security. There are plenty of opportunities to battle on domestic political issues linked to the future of the American economy. With our country facing the dual threats of unemployment and a growing federal debt bomb, we anticipate significant conflict between Democrats and Republicans. It is, however, in the national interest to ratify New START.

The writers were secretaries of state for the past five Republican presidents.