Russian aggression in Ukraine and nuclear saber rattling are jeopardizing the very global nonproliferation efforts that this week's Nuclear Security Summit in Washington seeks to further. Moscow's actions deserve a stronger response than they have received, not least to deter potential proliferators and reduce the risk of nuclear terrorism.

In some respects, Russia has been a responsible nuclear steward. Contrary to fears, no former Soviet nuclear weapon is known to have gone missing. During and after the Soviet collapse in 1991, Russia, with U.S. help, removed nuclear weapons from every former Soviet republic. The Nunn-Lugar Cooperative Threat Reduction Program and other U.S. initiatives have helped Russia dismantle nuclear missile silos, bombers and submarines; improve the security of nuclear weapons; and install technology to detect nuclear smuggling. The scale of these U.S.-Russian cooperative endeavors has only one parallel—U.S. aid to the Soviet Union under the Lend-Lease Act, which was the principal means for providing aid to foreign countries during World War II. Today, Russia and America co-lead the Global Initiative to Combat Nuclear Terrorism, a voluntary partnership of eighty-six nations and five international organizations, and have concluded accords to reduce strategic nuclear forces.

Since Vladimir Putin returned to the presidency in 2012, however, Russia's record in lessening nuclear dangers has been checkered. Moscow played a key role in the P5+1 talks held in November, which reached a historic deal to constrain Iran's nuclear weapons potential. On the negative side, Russia has flight-tested at least one missile banned by the Intermediate-Range Nuclear Forces Treaty.

Moscow's greatest nuclear transgression concerns Ukraine. To help quell serious debate in Ukraine about the risks of future Russian aggression, Moscow, London and Washington signed the Budapest Memorandum of Security Assurances in 1994. The three pledged to respect the independence, sovereignty and existing borders of Belarus, Kazakhstan and Ukraine, and refrain from the threat or use of force against them. Separately, France and China offered similar assurances.

In December 2014, Ukrainian President Petro Poroshenko said it would be “irresponsible” for his country to seek nuclear weapons, but many political and diaspora leaders wish Ukraine had retained former Soviet nuclear weapons. This would have been dangerous. Ukraine lacks the capability to maintain such weapons, which is vital for preserving safety. Kyiv would have had difficulty establishing survivable basing and secure command and control, thereby enhancing preemption risks. Russia's cyber espionage and jamming of Ukraine's wartime communications illustrate some of the challenges. There was also the hazard of Russia penetrating Ukraine's security and military sectors....

The remainder of this commentary is available on nationalinterest.org.

William Courtney, adjunct senior fellow at the nonprofit, nonpartisan RAND Corporation, was U.S. ambassador to Kazakhstan, Georgia and a U.S.-Soviet commission to implement the Threshold Test Ban Treaty. John Herbst, director of the Dinu Patriciu Eurasia Center at the Atlantic Council, was U.S. ambassador to Ukraine and Uzbekistan.

Commentary gives RAND researchers a platform to convey insights based on their professional expertise and often on their peer-reviewed research and analysis.