The U.S. assumed joint ownership when it short-circuited promising peace talks early on. From Ted Snider at antiwar.com:
On February 24, 2022, Russia illegally invaded Ukraine. From that moment on, every horror that would never have happened had Russia never gone to war was Russia’s responsibility.
But somewhere in an increasingly significant period between March and April, the US assumed joint responsibility.
There is an increasing pattern of proof that, before the war escalated, before tens of thousands of people died, before Ukraine’s infrastructure was devastated, the war could have been ended on terms acceptable to Ukraine and Russia. There is an increasing pattern of proof that, as Russia had planned, the war could have been quick and confined. Ukraine would promise not to join NATO, the Donbas would be autonomous as promised by the Minsk agreements, and Russian troops would leave without massively targeting civilian infrastructure and destroying the country.
But the US and the UK obstructed that outcome and stopped it from happening. The war could have been stopped, but it continued. That did not free Russia from responsibility. But, from that moment on, the US assumed shared responsibility.
The next phase of the war continued in the service, not of Ukrainian interests, but of US interests. It escalated and devastated the infrastructure and people of Ukraine.
Wars are illegal and immoral whether short or long. But, in the early days of the war, Russia seems not to have been targeting civilians or civilian infrastructure. A senior analyst at the Defense Intelligence Agency leaked to Newsweek that, in the first month of the war, “almost all of the long-range strikes have been aimed at military targets.” A retired Air Force officer, working with a “large military contractor advising the Pentagon,” told Newsweek that “the Russian military has actually been showing restraint in its long-range attacks.” The sources told Newsweek that Russia was not bombing indiscriminately and that the US dropped more missiles on the first day in Iraq in 2003 than Russia dropped in the first 24 days in Ukraine. “The vast majority of the airstrikes are over the battlefield, with Russian aircraft providing “close air support” to ground forces. The remainder – less than 20 percent, according to U.S. experts – has been aimed at military airfields, barracks and supporting depots.” The DIA analyst concluded that “that’s what the facts show. This suggests to me, at least, that Putin is not intentionally attacking civilians. . . . I know that the news keeps repeating that Putin is targeting civilians, but there is no evidence that Russia is intentionally doing so.” A senior DIA official says that “in terms of actual damage in Kyiv or other cities outside the battle zone, and with regard to the number of civilian casualties overall, the evidence contradicts the dominant narrative.”