Europe, you have been warned. President Vladimir Putin has waged a full-scale war against Ukraine for nearly four years and this week threatened that Russia was “ready right now” for war with Europe if need be. President Donald Trump has demonstrated that the US is ready to sell out Ukraine for the sake of a dirty deal with Putin’s Russia. His new US National Security Strategy prescribes “cultivating resistance to Europe’s current trajectory within European nations”. How much more clarity do you need?

Now it’s up to us Europeans to enable Ukraine to survive armed assault from Moscow and diplomatic betrayal from Washington. In doing so, we also defend ourselves. For a year now, people have been telling me that Trump will eventually get tough on Russia. It’s been the geopolitical version of Waiting for Godot. Then his personal real-estate emissaries come up with a 28-point “peace plan” that is a Russian-American imperial and commercial deal at the expense of both Ukraine and Europe.

European leaders jump into their familiar Trump-management mode, whittling away the most outrageous points by track-changes diplomacy to produce a version that – predictably enough – Russia in turn finds unacceptable. Although this 28-point plan only lasted a few days, it should long be studied as a historical document. It reveals just how far Trump’s US is prepared to go in reverting to a politics of empires and spheres of influence, over the heads of all Europeans. The old Polish rallying cry Nic o nas bez nas! (nothing about us without us!) must now go up from the whole of Europe.

Two questions follow. First, can Europe, together with like-minded countries such as Canada, sufficiently strengthen Ukraine and weaken Russia for the former eventually to prevail? Second, will it?

The answer to the first is that it will be very difficult, but we still can. If, at their summit on 18 December, EU leaders agree a way to use the frozen Russian assets held in Belgium, the gaping hole in Ukraine’s budget can be filled for at least the next two years. Europe’s combined economy is 10 times the size of Russia’s. European defence production is being ramped up. The list of military essentials that only the US can supply is getting shorter and Trump’s profit-seeking logic means most can still be bought. Germany, Poland, the Netherlands, Norway and Canada recently agreed to purchase another $1bn of US weapons for Ukraine. If Trump were again to cut the supply of US intelligence, trying to blackmail Ukraine into accepting a capitulation peace, that would be a major blow, but Ukrainian and European intelligence can already plug some of the gaps.

Ukraine itself has vital homework to do. The departure of president Volodymyr Zelenskyy’s right-hand man Andriy Yermak, in a major corruption scandal, creates an opportunity for Ukraine to make a bold domestic reset, perhaps in the form of a genuine government of national unity. The yellow-and-blue line of soldiers at the front is getting desperately thin. Since February 2022, prosecutors have opened almost 300,000 cases relating to absence without leave or desertion, and one does meet a lot of Ukrainian men of military age outside the country.

But Russia has growing problems of its own. Graveyards are reportedly being enlarged to accommodate at least 250,000 war dead and, with perhaps a further 750,000 wounded, recruitment becomes difficult even for a dictatorship that has a much larger population than Ukraine. The economy has held up remarkably so far, thanks to a “war economy” boost and flourishing ties with China and India (witness this week’s Delhi love-in between Putin and Indian prime minister Narendra Modi). But inflation is soaring, interest rates are above 16% and, crucially, the price of oil is declining. Long-range Ukrainian attacks have damaged more than a third of Russia’s oil refineries. Some 80% of Russia’s seaborne oil exports go through the Danish straits in “shadow fleet” ships that generally don’t meet international safety and environmental standards. Europe could slow this revenue flow by stopping and rigorously inspecting those ships.

If Europe can generate sufficient military and economic support for Ukraine, and economic pressure on Russia, then at some point in 2026 or 2027 the incentive structure for Putin would change. His generals would tell him “we’re not getting anywhere” and his central bank would tell him “the economy is cracking”. A ceasefire along the existing frontline then becomes more likely. It’s hard to envisage any formal peace treaty that both Putin and Zelenskyy could agree to sign, but a longer-term truce is a realistic possibility.

Europe would then face a further challenge. Who has won this war will be decided not in that moment, when the guns fall silent, but over the five to 10 years that follow. If in 2030, as well as occupying and russifying Ukrainian territory larger than the size of Portugal and Slovenia combined, Moscow can privately boast that the rest of Ukraine is insecure, dysfunctional, demoralised, depopulated and subject to strong Russian influence, then Russia will have won. If in 2030 most of Ukraine is sovereign, secure, a “steel porcupine” capable of deterring any future Russian attack; if it has a dynamic economy, attracting foreign investment, offering good jobs to veterans and persuading young Ukrainians to come home from abroad; if it further has a halfway decent democracy, a strong civil society and is seriously on track to become a member of the EU; then Ukraine will have won. But that will require a sustained and substantial effort from Europe, as well as from the Ukrainians themselves.

Yes, Europe can do it. But will it? I can offer you a long list of reasons why it may not. The still pervasive myth of Russian invincibility. Learned helplessness after 80 years of depending on the US for our security. The procedural slowness of the EU. Acute competition for public money in often heavily indebted European states with ageing populations that have unrealistic expectations of what those states can provide. The kind of politics that brings Germany’s governing coalition to the brink of collapse over a proposed modest trimming of a state pension scheme that already devours a quarter of the federal budget. The national egoisms that have seen Belgium’s prime minister hold out against the seizure of the Russian frozen assets and France squabble with Germany over a supposedly joint project for a next-generation fighter jet. Need I go on?

Yet against this pessimism of the intellect I place the optimism of the will. For that is the one thing that can turn “Europe can” into “Europe will”. Willpower. Strategic determination. Fighting spirit. The courage to put the long-term collective interest before short-term party-political opportunity. We know that individual nations have done extraordinary things against the odds in moments of existential danger: Britain in 1940, Ukraine in 2022. But will our diverse, complicated, self-doubting continent rise to this major but still significantly less extreme challenge? Europe can if it wills it.