The poison injected into international life by Russia’s intervention in Ukraine spreads apace. When Russia joined the G8 and the G20 in the late 90s, the idea was to integrate Moscow into the collaborative systems linking advanced states, an integration expected to be as political as it was economic. Now these encounters are more likely to be the scene of confrontation than of cooperation. This weekend’s G20 summit in Australia could well be dominated by Ukraine, with the less engaged powers standing by bemusedly as the Europeans, the Americans and the Russians trade accusations and counter-accusations.

They were already exchanging the opening shots before the meeting began. David Cameron warned of increased sanctions in Brisbane on Friday, while President Vladimir Putin, speaking in Moscow before leaving for the summit, said the United States, by imposing sanctions on Russia, was “crudely violating” the principles of the trade institutions it played a leading part in creating. Russia, he added, could weather sanctions, which would hurt those who imposed them as much as they did Russia.

The way Mr Putin discusses these matters is as if Russia were quietly minding its own business when it was suddenly subject to unprovoked acts of economic warfare. He and his foreign minister, Sergei Lavrov, are masters of the art of feigned innocence. They intersperse bouts of the most brazen lying with periods in which they appear open to reason. They send in the troops, then pull them out. They endorse agreements, then renege on them.

That is why the war in Ukraine has not gone away. Sustained by Russian troops on the ground and by Russian lies in the media, it follows what is now a familiar and deadly script. First there are incursions by Russian forces, going into Ukraine to reinforce the rebels there and assist them in consolidating or extending the territories they control. Then there is fighting. Sometimes the Ukrainians gain the upper hand, in which case the Russians send in more troops to redress the balance. Then comes a sort of diplomatic patching up, the latest version of which was the Minsk accord of early September. The Ukrainians are so desperate for any respite and the Europeans are so anxious to believe that there is, there could be, or at least that there should be, a chance of a settlement that they go along with it.

Then the violations begin, and then they get worse, which, according to the Ukrainians, the Organisation for Security and Co-operation in Europe, and now Nato, is what has been happening in recent weeks as Russian columns move once more into the country. The motive can be guessed at: the rebel zones don’t make much economic or political sense, nor does a Crimea reachable only by ferry. The idea may be to carve out something more “logical”.

No doubt the Ukrainians also violated the ceasefire, as well as responding to rebel moves. But an end to the fighting would have been so overwhelmingly to Ukraine’s advantage, even if areas remained under rebel control, that it is simply not believable that they were the main instigators. The plain truth is that Russia will not let Ukraine go. It is waging a hybrid war, part conventional but deniable (at least by Russian standards), and one camouflaged by a huge campaign of disinformation, in Russia itself, in Ukraine, and in the rest of the world. Sanctions are the only available response. They will have to be maintained and very possibly increased, and the same goes for economic aid to Ukraine.

The Russians might withdraw the forces they have sent in. They have done that before, but unfortunately this has never been the end of the story. Indeed, that story could be a long and damaging one for both sides. The latest collateral damage is to the programme to ensure the security of nuclear materials in Russia, one on which the United States and Russia have collaborated for 20 years. It was based on a common understanding that such materials were too widely dispersed and therefore vulnerable to seizure by terrorists. That remains true, but now the Russians have announced that they foresee no new joint projects in Russia. As with so many other areas in relations with Russia, the problem is not over, but the era of co-operation unfortunately is.