The mass graves of civilians murdered and mutilated in Bucha, Ukraine, are stark evidence of the horrors of war - and of war crimes. Putin's invasion of Ukraine - itself a violation of international law - raises a profound challenge to the world. How can a dictator armed with nuclear weapons be held accountable for the crimes of war?

Ukraine is not a member of NATO. NATO nations have no commitment to its defense. Despite this, President Biden has rallied the NATO allies to impose unprecedented economic sanctions on Russia, to help supply Ukrainians with armaments and with humanitarian aid, and to shelter the millions of Ukrainians dislodged from their homes by the war. These measures are not without cost and risk. Russia supplies a large portion of Europe's oil and natural gas, and Russia and Ukraine a large portion of the world's wheat supply. As the sanctions have attached, the cost of food and fuel has soared across the world.

As the destruction wrought by war grows and the stirring Ukrainian resistance and horrible human suffering touch anyone with a heart, the pressure to escalate grows. From the start, President Biden and the NATO allies ruled out direct military intervention of U.S. or NATO forces. Enforcing a no-fly zone over Ukraine - which would put U.S. planes in direct conflict with Russian planes - has been ruled out also. Supplying Ukraine with jets from Poland and elsewhere was also rejected.

Some argue that Putin will take advantage of this caution to crush Ukraine. Yet, to date, Ukrainian resistance has frustrated the Russian military. Now it appears that the Russians are withdrawing from around the Ukrainian capital and focusing their forces in the east - the Dombas region that is largely Russian speaking and historically more sympathetic to the Russians. As the war grinds on, the terrible toll in lives and destruction continues to rise in Ukraine.

Some would argue that it is immoral for the U.S. and NATO to stand by as Russian military forces savage Ukraine. The moral calculation for U.S. policy, however, is clear. Direct intervention in the war would risk a nuclear conflagration. That would cause far more death and destruction across the world - and in Ukraine - than the current horrors. The problem with exchanging an eye for an eye is that we all end up blind.

How then can Putin be held accountable for his illegal aggression and for the war crimes committed by Russian forces? Continued aid and support for the Ukrainians is vital. The higher the price the Russian military pays in Ukraine, the less likely Putin will be emboldened to repeat his aggression.

Sanctions are an imperfect response since the Russian people who pay the price are not those who made the decisions. So long as Russian forces wage war on Ukraine, however, the sanctions should continue - and continue to be tightened. The U.S. and Europe should join in efforts to ensure that the poor at home and across the world are shielded to the extent possible from the economic fallout.

International law must be reasserted. International tribunals - in the International Criminal Court, at the UN General Assembly - and independent public tribunals should indict Putin's aggression and detail the war crimes. Then the tribunals - or the allied nations - can target individual sanctions on those responsible, making them pariahs across the world. Those nations - from China to India to many countries across the world - who have remained "neutral" should be challenged to reassert their commitment to non-intervention and to the laws of war.

Hopefully, Ukrainian resistance will force a negotiated settlement. Inevitably, any compromise agreement will appear to reward Putin for his aggression. The world community must ensure that whatever the agreement, the crimes are exposed, and the commitment to non-intervention and international law is re-asserted. Putin's nuclear arsenal may shield Russia from attack, but it cannot shield him and his associates from accountability.

You can write to the Rev. Jesse Jackson in care of this newspaper or by email at jjackson@rainbowpush.org. Follow him on Twitter @RevJJackson.