Last week, doom-scrolling took on a new meaning as unspeakable images crowded our social media screens, day by day, following the withdrawal of Russian forces from the suburbs of Kyiv. We tried not to look. War should not be a spectator sport. How would we feel if our own ruined bodies were displayed like this alongside adverts for sofas and cars? But it felt equally wrong to turn away. Social media has made us intimate witnesses to the darkest passions of the human animal.

These scenes will remain imprinted on our collective memories for decades. In Motyzhyn, the tortured body of the local mayor, Olga Sukhenko, her husband Igor and son Alexander lying partially exposed in a shallow grave. In Bucha, the man lying dead beside his bicycle while his dog stands guard. Men with their hands bound while they were executed; obscene debris of humans partially incinerated; people herded into death cellars and pits. And, yes, video evidence of Ukrainian soldiers apparently killing Russian captives in cold blood after an ambush.

War is hell, and atrocities happen on both sides. That is precisely why those who start wars of aggression must be held responsible for the consequences. They brutalise invader and defender alike. In April and May 1945, the allied liberators of Berlin committed war crimes, rape and murder, on a vast scale - it's just that there were no phones to record them. They escaped with impunity in the interests of the greater cause of defeating fascism.

The names of Bucha and Mariupol are now synonymous with the worst state-sponsored barbarism seen in Europe since the Second World War. Russian atrocities may not yet be on the same scale as the Nazi crimes at Buchenweld or Oradour-sur-Glane, but it is only a matter of degree. Hitting crowded train stations with cluster bombs is not far off. This evil, there is no other word for it, has rightly shocked the world, but it is of especial significance to us here in Europe because it is on our doorstep.

It was President Zelensky who said last week that “the architecture of global security has been fatally undermined”. As guarantors of that security, the West shares responsibility for its destruction, and Europe most of all. We deluded ourselves that this kind of thing could not happen here. Prince William remarked as much recently, in much criticised statements prior to his visit to Jamaica. But he was only telling the truth. All of us believed that Europe was somehow different from Iraq, Iran, Syria. Our “rules-based” institutions, like the United Nations, were supposed to have made war unthinkable here at least..

That era ended in Bucha last week. One of the first casualties has been the UN itself. It has morally imploded, much as its predecessor the League of Nations imploded in the face of the Nazi occupations in the 1930s. It too was dedicated to preventing wars through collective security and disarmament – right up until the moment it couldn't. The League's last act, ironically, was to expel Russia after its invasion of Finland in 1939. The United Nations was not even prepared to do that last week.

President Zelensky did not hold back in his excoriating address to the UN Security Council last week. “Has the time of international law ended?”, he asked. “Where is the security that the Security Council is supposed to guarantee?...The right of veto should not be seen as a right to kill”. If you cannot condemn aggression you might as well “dissolve yourself now”. In a sense, by its own inaction, it already has.

Every article of the UN charter has been violated by a member of the UN Security Council, Russia. The invasion of a sovereign nation, the deliberate targeting of civilians, ethnic cleansing, torture, rape, looting, starvation. The Russian ambassador, Vasily Nebenzya, calmly blaming the Ukrainians for the butchery and mayhem in their own country was final nail in the coffin of the United Nations as it currently exists. What comes next, no one knows.

It is not only the UN that has been undermined. The European Union was slow to act over the invasion of Ukraine, duped by Vladimir Putin into believing it would never happen. When the war started, Brussels found that it did not initially have the legal means to send even defensive arms to Ukraine. The EU is not a military organisation, after all, but essentially a trading standards bureaucracy with a human rights annex. No one ever imagined that the EU would go to war. It is not a state.

Yet until last month the European Union was regarded by many, including the celebrated US academic Francis Fukuyama, as a kind of beacon of peaceful coexistence. It was celebrated as an institution that had helped put an end to war by linking Germany and France in economic bonds of mutual advantage. This was also the thinking behind Ostpolitik, the now discredited EU policy of trying to envelop Russia and Eastern Europe in similar networks of economic association. Liberal capitalism would, it was hoped, give them an offer they couldn't refuse: economic prosperity in exchange for peace.

That worked out well. It now seems like madness for Germany to have allowed itself to become dependent on Russian oil and gas, gifting its economic security to the Kremlin mafia. But they had their reasons. Germany above all has borne the burden of history these last 80 years as the incubator of the Nazi evil that gave us the Holocaust and the World War Two. Germany saw itself as guarantor of the Great Delusion: the belief that ward could be outlawed by rules. How grimly ironic that Germany should now be largely responsible for Europe financing Vladimir Putin's war machine to the tune of a billion euros a day through purchasing Russian hydrocarbons.

European countries had outsourced their defence to Nato, another international organisation that has had its architecture undermined by the war. Nato's philosophy of nuclear deterrence, Mutual and Assured Destruction, dates from the Cold War and is clearly unsuited to combatting the conventionally-armed neo-imperialism of Russia and China. Avoiding nuclear war is clearly not enough.

It was left to individual countries, like Britain, to arm the Ukrainians when the EU and Nato stood by. America and Britain then led the move to impose sanctions on the Russian central bank and exclude it from the Swift banking payments system. Now the Czech Republic has shamed Nato by sending some out-dated tanks to Ukraine. Nato must now meet its responsibilities to defend democracy and global security.

The next four weeks will be crucial. The Russians are redeploying their troops and armour in a bid to sever Ukraine, secure the Donbas region and encircle and destroy the Ukrainian army. It will be a battle the like of which we haven't seen since 1944, and comparisons are being drawn with Stalingrad. This is not just a battle for a region or a country, but for civilisation itself. The heroes of the Ukrainian resistance have been fighting for all of us. Amazingly, they have stopped Putin's war machine in its tracks. We must now give them the guns and ammunition to finish the job.