The United States and other NATO nations should send lethal defensive weapons to Ukraine to help that country better protect itself against Russian-backed rebels, as well as to convince Russian President Vladimir Putin that his illegal, immoral aggression will not go unchecked by the West.

To be sure, Western nations have taken Putin's moves seriously and have responded with relatively effective economic sanctions. Combined with the collapse of the price of oil, the sanctions have sent the Russian economy reeling.

But clearly this is not enough. The separatists, aided by Russian troops that NATO estimates at up to 1,000 and Ukraine estimates at up to 9,000, are on the march. The results are tragic. Despite a cease-fire deal signed in September, there have been an estimated 5,100 killed since April. And in the past four months alone, NATO estimates that separatists have captured about 193 square miles in portions of eastern Ukraine.

To date, the Obama administration has agreed to send only nonlethal aid, such as body armor, night-vision goggles and other equipment to the Ukrainian military. Russia reportedly has not hesitated to send heavy weapons, including the surface-to-air missile system that allegedly downed a Malaysia Air jet in July.

According to the New York Times, the Obama administration is reconsidering its reluctance. Gen. Philip M. Breedlove, NATO's military commander, now may be in favor of sending defensive weapons, and he may have allies in Secretary of Defense Chuck Hagel, Secretary of State John Kerry and others.

All involved would be well-served by heeding a report released on Monday from eight former defense and diplomatic officials with direct experience with Russia, Ukraine and Europe. Among their clear conclusions is that the United States should send Ukraine $1 billion in military assistance in each of the next three years and that other NATO nations should similarly send military aid.

This does not mean sending troops. Nor does it suggest that there is a decisive military solution to the ongoing crises. In fact, Ukrainian President Petro Poroshenko has said that there is only a diplomatic solution. But that won't happen until the rebels have an incentive to negotiate. Right now they are winning and may achieve their — and Putin's — objectives through brutal military means.

This isn't only about Ukraine. Putin has menaced neighboring nations with his revanchist policies, and the West's current timid course may embolden him to make a military miscalculation that could spark a broader war, especially if he moved on Baltic members of NATO.

And it's not even only about Europe. Negotiations with Iran and North Korea over their potential and existing nuclear weapons programs could be made more difficult. The example from Kiev is not a good one. In the post-Soviet era, Ukraine voluntarily gave up its nuclear weapons in exchange for international agreements guaranteeing its sovereignty. Russia has clearly violated that agreement, first by its illegal annexation of Crimea and now by destabilizing other portions of Ukraine. The theocracy in Tehran and the opaque regime in Pyongyang are clearly watching these events, and they may be seeing all the more reason to seek or maintain a nuclear arsenal.

In its conclusion, this week's report spells out the stakes of inaction: "The United States and NATO must respond, both to support Ukraine and to push back against Russia's unacceptable challenge to the postwar European security order. This will require more military assistance, some of it lethal, but none of it offensive. Should we delay action, the West should expect that the price will only grow. Should we not act more robustly, we can expect to face further Russian incursions, possibly including attempts to redraw borders elsewhere, and efforts to intimidate former Soviet states into accepting Russian dominance."

Russia is counting on the West to hesitate. The West cannot afford to. It should consider levying even stricter sanctions. And just as the United States and the European Union tried to deter Russia and the Ukrainian separatists economically, it should do so militarily by sending lethal defensive aid, giving the preferred method of crisis resolution, diplomacy, a fighting chance.