Of all the threats to global security and peace, the most dangerous is the proliferation and potential use of nuclear weapons. That’s why, seven years ago in Prague, I committed the United States to stopping the spread of nuclear weapons and to seeking a world without them. This vision builds on the policies of presidents before me, Democrat and Republican, including Ronald Reagan, who said “we seek the total elimination one day of nuclear weapons from the face of the Earth.”

Thursday in Washington, I’ll welcome more than 50 world leaders to our fourth Nuclear Security Summit to advance a central pillar of our Prague Agenda: preventing terrorists from obtaining and using a nuclear weapon. We’ll review our progress, such as successfully ridding more than a dozen countries of highly enriched uranium and plutonium. Nations, including the United States, will make new commitments, and we’ll continue strengthening the international treaties and institutions that underpin nuclear security.

Given the continued threat posed by organizations such as the terrorist group we call ISIL, or ISIS, we’ll also join allies and partners in reviewing our counterterrorism efforts, to prevent the world’s most dangerous networks from obtaining the world’s most dangerous weapons.

Beyond preventing nuclear terrorism, we’ve made important progress toward the broader vision I outlined in Prague.

First, we’re taking concrete steps toward a world without nuclear weapons. The United States and Russia remain on track to meet our New START Treaty obligations so that by 2018 the number of deployed American and Russian nuclear warheads will be at their lowest levels since the 1950s. Even as the United States maintains a safe, secure and effective nuclear arsenal to deter any adversary and ensure the security of our allies, I’ve reduced the number and role of nuclear weapons in our national security strategy. I also have ruled out developing new nuclear warheads and narrowed the contingencies under which the United States would ever use or threaten to use nuclear weapons.

Second, we’re strengthening the global regime – including the Nuclear Non-Proliferation Treaty – that prevents the spread of nuclear weapons. We’ve succeeded in uniting the international community against the spread of nuclear weapons, notably in Iran. A nuclear-armed Iran would have constituted an unacceptable threat to our national security and that of our allies and partners. It could have triggered a nuclear arms race in the Middle East and begun to unravel the global nonproliferation regime.

After Iran initially rejected a diplomatic solution, the United States mobilized the international community to impose sanctions on Iran, demonstrating that nations that fail to meet their nuclear obligations will face consequences. After intense negotiations, Iran agreed to a nuclear deal that closes every single one of its paths to a nuclear weapon, and Iran is now being subjected to the most comprehensive inspection regimen ever negotiated to monitor a nuclear program. In other words, under this deal, the world has prevented yet another nation from getting a nuclear bomb. And we’ll remain vigilant to ensure that Iran fulfills its commitments.

Third, we’re pursuing a new framework for civil nuclear cooperation so countries that meet their responsibilities can have access to peaceful nuclear energy. The international fuel bank that I called for seven years ago is now being built in Kazakhstan. With it, countries will be able to realize the energy they seek without enriching uranium, which could be at risk of diversion or theft.

Our progress notwithstanding, I’m the first to acknowledge that we still have unfinished business. Given its violations of the INF Treaty, we continue to call on Russia to comply fully with its obligations. Along with our military leadership, I continue to believe that our massive Cold War nuclear arsenal is poorly suited to today’s threats. The United States and Russia – which together hold more than 90 percent of the world’s nuclear weapons – should negotiate to reduce our stockpiles further.

The international community must remain united in the face of North Korea’s continued provocations, including its recent nuclear test and missile launches. The additional sanctions recently imposed on Pyongyang by the United Nations Security Council show that violations have consequences. The United States will continue working with allies and partners for the complete and verifiable denuclearization of the Korean Peninsula in a peaceful manner.

More broadly, the security of the world demands that nations – including the United States – ratify the Comprehensive Test Ban Treaty and conclude a new treaty to end the production of fissile material for nuclear weapons once and for all.

I said in Prague that achieving the security and peace of a world without nuclear weapons will not happen quickly, perhaps not in my lifetime. But we have begun. As the only nation ever to use nuclear weapons, the United States has a moral obligation to continue to lead the way in eliminating them. Still, no one nation can realize this vision alone. It must be the work of the world.

We’re clear-eyed about the high hurdles ahead, but I believe that we must never resign ourselves to the fatalism that the spread of nuclear weapons is inevitable. Even as we deal with the realities of the world as it is, we must continue to strive for our vision of the world as it ought to be.