President Obama has repeatedly described Russia’s annexation of Crimea as illegal and illegitimate, but he also has sought to minimize the strategic significance of Vladimir Putin‘s land grab. In fact, Moscow’s actions—including threatening “civil war” if Ukraine resists the orchestrated seizures of government buildings and uprisings in eastern Ukraine by ethnic Russian separatists this week—are more than isolated instances of law breaking. Russia’s behavior, and its legal and institutional justifications, are dangerously destabilizing the existing international system. What is the likely result? The use of force around the world will be encouraged, and the incentive to acquire nuclear weapons magnified.

The three basic principles of international law, reflected in the United Nations Charter and long-standing custom, are the equality of all states, the sanctity of their territorial integrity, and noninterference by outsiders in their internal affairs. Yet Moscow now insists that it has unique rights and privileges to protect the interests of Russian-speaking populations outside its borders and has special prerogatives regarding “historically Russian” territories that were not included in the Russian Federation upon the Soviet Union’s dissolution in 1991.

David Gothard

Regarding Ukraine, these claims have been translated into a set of specific demands that Moscow has made in speeches by Mr. Putin and others, in articles sanctioned by the Kremlin, and in discussions between Russian Foreign Minister Sergei Lavrov and Secretary of State John Kerry. The demands include that Ukraine postpone its planned May elections, change its constitution to provide for regional autonomy (making eastern Ukraine more vulnerable to Russia’s capture), and dramatically weaken the national government in Kiev. Moscow insists that Ukraine make Russian the country’s second “official” language and ban certain nationalist political parties.

Moscow also says Ukraine must become a neutral, non-allied and essentially demilitarized state—a status known during the Cold War as “Finlandization,” after terms that the Soviet Union imposed on Finland as the price of its “independence.”

Mr. Putin’s demands clearly violate the principle of nonintervention in internal affairs enshrined in the U.N. Charter and customary international law.

Moscow’s use of troops that have removed their Russian insignia, coupled with explicit denials that its military forces were even engaged in operations in Crimea, violates the Geneva Conventions. The failure to promptly repatriate captured Ukrainian troops and equipment after the invasion of Crimea was complete, and ended, and Russia’s attempt to coerce Ukrainian soldiers to join the Russian military, are also major violations.

The laws of war are already under assault from terrorist organizations, whose fighters routinely operate out of uniform to blend into the civilian population. Having a major power like Russia engage in similar conduct further erodes respect for these vital norms—and encourages such rogue behavior by other governments and by rebel movements.

Moscow’s disregard of its treaty commitments has also gravely undermined the cause of nuclear nonproliferation. In particular: The takeover of Crimea shreds the 1994 Budapest Memorandum on Security Assurances, by which Ukraine agreed to give up its formidable nuclear arsenal in exchange for commitments from Russia, Britain and the U.S. to respect its political independence and territory.

Now Russia has demonstrated that military force in general, and nuclear weapons in particular, may well remain the only reliable means of protection against hostile actions by larger, more powerful states. If the Russian takeover of Crimea continues to meet with only a tepid international response, the message is clear: Security commitments among states are worthless. This development is certain to have profoundly destabilizing consequences world-wide.

Thus it is hard to comprehend the Pentagon’s announcement Tuesday that the U.S. would drastically reduce its nuclear-weapons capability to comply with the New START treaty with Russia. America is reducing its ability to defend itself in order to honor a treaty with a country that has just flagrantly violated a treaty.

In the event that the U.S. and its allies decide to abandon the minimalist and ineffective approach they have taken so far, several options for challenging Russia’s bogus claims come to mind. These measures are not a substitute for strong leadership, but they at least offer the prospect of countering what so far has been a one-sided conflict.

As a start, the Obama administration should seek a U.N. General Assembly resolution requesting the International Court of Justice’s opinion on the legality of the Russian annexation of Crimea. Russia would have no veto over such a request. Since the General Assembly has already voted overwhelmingly to declare the annexation illegal, this should easily be achieved. If the International Court of Justice concurs that the annexation is illegal, that would eviscerate Moscow’s bogus international-law arguments and could serve as the basis for future legal claims against Russia and Russian entities.

The U.S. and its allies should also challenge the legality of Russia’s actions in every conceivable legal venue, whether domestic or international. Since Moscow has justified its annexation by claiming Ukrainian governmental corruption and repression against Russian speakers, Western governments should give high-profile publicity to whatever evidence of Russian official corruption they possess, and to evidence that unrest in Crimea and eastern Ukraine has been fostered by Russian military and intelligence agents.

Nongovernmental organizations, which cast themselves as guardians of the international order, have a role to play in condemning and challenging in courts of law and in public opinion Russia’s actions against Ukraine. This would also offer NGOs the opportunity to show their neutral commitment to maintaining the international order, as many of these groups claimed to be doing in challenging the legality of American actions in Vietnam, Afghanistan, Iraq and the Arabian Peninsula.

More than arcane legal principles are at stake. Western failure to champion a narrative of international rights and wrongs, rooted in the language of law and legitimacy, would be tragic. Meeting Russia’s aggression with passivity undermines already weakened domestic support for a robust and engaged foreign policy in the U.S. and other Western countries, and it promises to make the world a more lawless and violent place.

Messrs. Rivkin and Casey served in the Justice Department during the Reagan and George H.W. Bush administrations. They are partners in the Washington, D.C., office of Baker Hostetler LLP. Mr. Rivkin is also a senior adviser to the Foundation for the Defense of Democracies.