In a way, the Ukranian president, Volodymyr Zelenskiy, ruined the Americans’ plans. Early this year, when American intelligence concluded that Vladimir Putin would indeed invade Ukraine, no one expected it to be a long war. The conventional wisdom, shared by western analysts and Putin himself, was that Ukraine would rapidly fall to Russia’s military, and that after some strong words and a tepid round of sanctions, the US and Nato would cut a deal with Russia. They would give Putin hegemony over much of Ukraine – the eastern part, if not the whole thing – and what was left over would get an aspirationally democratic government with a European orientation. This detente would last until Putin got greedy again, at which point the cycle of Russian expansion and western accommodation would resume, this time a little further to the west. At some point in that version of events, Zelenskiy himself would have been arrested by the Russians. And then he’d be gone.

Instead, the man who addressed Congress on Wednesday morning did so from a besieged and defiant Kyiv, shelled and bombed nightly by the encircling Russian forces but still in Ukrainian hands. Zelenskiy appeared via video on a large screen at the front of a room full of senators and congresspeople in blue velvet chairs. He was unshaven. He looked unslept. By remaining in Kyiv, even as Russian troops encircle it and Russian bombs fall from the sky, Zelenskiy has made himself into a symbol of the Ukrainian people, whose surprising courage, determination, and defiance in the face of the Russian aggression have called the west’s moral bluff. Their stand, and Zelenskiy’s stand, have transformed what could have been a cynical calculation about how to manage Russia’s renewed imperial ambitions into a more meaningful – and more challenging – test of the west’s pro-democracy values. Days ago, when Americans offered Zelenskiy a discreet and secure exit from Kyiv, he reportedly told them: “I need ammunition, not a ride.”

This sounds like action movie dialogue, and at times Zelenskiy’s skillful public rhetoric has evoked his past life as a comedian and entertainer. He has come a long way from the days when his jokes included pretending to play a piano with his dick. But it is difficult to dismiss Zelenskiy’s words as mere macho posturing when one considers that the stakes are his own life. Russia’s lethal power is superior, and though the Ukrainians have mounted an impressive military and guerrilla resistance, it is unlikely that they will be able to permanently hold Kyiv unless something drastic changes in the war. Zelenskiy’s wife and two small children, who fled at the beginning of the invasion, will probably never see him again. His presidency has taken on the tenor of a suicide mission; he broadcast to Congress from the location in Kyiv where he is waiting to die. The contrast between what Zelenskiy has been willing to do for his country’s sovereignty, and what the western leaders on whom he is calling for help have been willing to do for their own professed values, is the source of Zelenskiy’s tremendous moral power. In his addresses to western leaders – he has also spoken to the Canadian and UK parliaments over the past days – Zelenskiy speaks in inspirational tones. But he is looking to evoke the west’s shame. On Wednesday, he spoke to Congress wearing a T-shirt in the dusty green of a military uniform. The American lawmakers who watched him wore suits.

In his address, Zelenskiy asked the Americans for the same things he has been asking for since the beginning of the Russian invasion: first, he wants a no-fly zone over Ukraine to stop the incessant and devastating bombing that has killed thousands of Ukrainians, including many civilians. Second, he wants the US to re-enter a deal that it backed out of this past week, in which America and Nato would facilitate the transfer to Ukraine from Poland of Soviet-made MiG military jets – planes that Ukrainian fighters know how to fly, and which could help make Russia’s air war over their country at least a little less asymmetrical. Third, he wants the west, and particularly the US, to continuously strengthen its sanctions on Russia. He called for individual restrictions to be placed on members of Putin’s government and their assets, and he called for American companies to cease operations in Russia. “Their market is flooded with our blood,” he said. He asked, too, for American ports to be closed to Russian-made goods. “Peace is more important than income.”

It is one of the most persistent and embarrassing themes of America’s status as a superpower that the US’s values frequently do not align with its interests – or, for that matter, with its capacities. Zelenskiy is likely to get little of what he asked for. A no-fly zone could only be enforced by shooting down Russian planes and taking out Russian air defense operations that are located not only in Ukraine, but also in Belarus and Russia. Putin would see this as an act of war, and the US and Nato would be pulled into a potentially world-ending global conflict. The transfer of planes also seems unlikely: the Pentagon recently rejected American participation in the scheme, fearing that it, too, would be interpreted as an unacceptable escalation by the Russians.

Harsher sanctions meant to cripple the Russian economy seem more likely, and like a less dangerous way to channel the considerable political and public support for the Ukrainian cause. The aim of such sanctions is not only punitive, but tactical. Despite the visibility of Russian dissidents in American media, the truth is that a large majority of Russians support Vladimir Putin, and support the war. Putin has the devoted support of the Russian people in no small part because they feel that their autocratic leader has secured Russia’s domestic stability and material prosperity. Sanctions could eventually change this calculation among ordinary Russians, potentially destabilizing Putin’s regime.

It will be tempting for many in America to justify their aversion to further military escalation in eastern Europe by denying the moral authority of the Ukrainian cause. Those on the Republican right, confused about their party’s relationship to the Putin regime, will try to paint Ukraine as a corrupt denizen of decadence. Those on the far left will point to the mujahideen in Afghanistan as an example of the dangers of American support for anti-Russian militias. But this rhetoric will largely function as a distraction from what is really happening in Ukraine. Zelenskiy is right that his people have a moral claim, as he said to Congress, to “Live in their own country and choose their own future.” He is right, too, that watching Putin deny Ukraine this right should make us all ashamed. But he is wrong to think that America has the power, really, to stop him.

• Moira Donegan is a Guardian US columnist