December 5 marks the 20th anniversary of the signing of the Budapest Memorandum on Security Assurances for Ukraine. Russia has grossly violated the commitments it made in that document. That imposes an obligation on Washington to support Ukraine and push back against Russia. This is not just a matter of living up to U.S. obligations. It is also about preserving the credibility of security assurances, which could contribute to preventing nuclear proliferation in the future.

As many analysts note, Vladimir Putin may have more at stake in Ukraine than does the West. The United States, however, has a strong interest. That stems not just from twenty-three years of bilateral relations with Kyiv but from U.S. commitments in the Budapest memorandum.

When the Soviet Union collapsed in 1991, nuclear arms lay in sites scattered across the former Soviet republics. Ukraine inherited the third largest nuclear arsenal in the world, including some 1,900 strategic nuclear weapons designed to strike the United States.

Nothing in the post-Soviet space commanded more attention from the Bush 41 and Clinton administrations than making sure that the Soviet Union’s demise did not increase the number of nuclear-armed states. Washington brokered with Kyiv and Moscow the terms under which Ukraine agreed to eliminate the strategic missiles, missile silos and bombers on its territory and transfer the 1,900 nuclear warheads to Russia for disassembly.

A key element of the arrangement—many Ukrainians would say the key element—was the readiness of the United States and Russia, joined by Britain, to provide security assurances. The Budapest memorandum committed Washington, Moscow and London, among other things, to “respect the independence and sovereignty and existing borders of Ukraine” and to “refrain from the threat or use of force” against that country.

The Kremlin has violated those commitments. Using soldiers in Russian combat fatigues without identifying insignia, whom Mr. Putin later admitted were Russian, Moscow seized Crimea in March.

Russia subsequently encouraged and armed separatists in eastern Ukraine. When the Ukrainian military appeared to gain the upper hand against the separatists, regular Russian army units entered Donetsk and Luhansk to support them.

What part of its commitments to “respect the independence” and “refrain from the threat or use of force” do the Russians not understand?

Since the Ukraine-Russia crisis erupted, Washington has provided political and economic support to Kyiv, as well as non-lethal military assistance. It has worked with the European Union to put in place increasingly tough sanctions on the Russian economy.

Washington should do more. It is time to provide the Ukrainian military defensive arms, such as light anti-tank weapons. That would raise the costs to the Russian army of any further fighting and help deter it. Washington should also be prepared, if Moscow does not alter course and facilitate a peaceful settlement, to work with Europe to impose additional economic sanctions.

These are actions that the United States owes Ukraine for giving up the nuclear arms on its territory. In 1994, Washington wrote Kyiv a check for U.S. support in the Budapest memorandum—albeit hoping that it would never be cashed. Unfortunately, it has.

This is not just a matter of assisting Ukraine in fulfillment of U.S. obligations. It is also about preserving the credibility of security assurances for the future.

Security assurances such as those in the Budapest memorandum do not carry as much weight as NATO security guarantees or the guarantees in the mutual security treaties that the United States has with Japan and South Korea. Still, security assurances have played a role in the effort to freeze and end North Korea’s nuclear program.

These kinds of assurances may not by themselves offer major leverage. However, when looking for ways to prevent nuclear proliferation, Washington and its partners should marshal every possible tool. The problem is that Russia’s actions against Ukraine have discredited security assurances.

If a North Korean diplomat were to ask his or her Ukrainian counterpart how the Budapest memorandum worked out, the response would not be a happy one. At a September conference in Kyiv, former President Leonid Kuchma, who signed the Budapest memorandum for Ukraine, said that Ukraine had been “cheated.” Prime Minister Arseniy Yatseniuk referred to the “notorious” Budapest memorandum. Such comments do not make good advertisements for future security assurances.

Washington cannot undo Russia’s violations. It can and should, however, do more to fulfill its obligations under the Budapest memorandum by doing more to bolster Ukraine and penalize Russia until Moscow alters its policy. Such U.S. action could also change the Ukrainian narrative in the hypothetical conversation with a North Korean diplomat to “the Russians violated the memorandum, but the Americans backed us to the fullest and made Moscow pay.” That would help restore credibility to security assurances as an element in the toolkit of America’s non-proliferation diplomacy.