Faltering public patience for key military and financial aid to Ukraine could unleash political turbulence, especially if inflation and other economic woes worsen, they say.

Other geopolitical tensions — for example between Washington and Beijing over Taiwan's independence — would likely further erode support for Ukraine.

That could lead to quiet but mounting pressure on Ukraine's President Volodymyr Zelensky to negotiate an end to the hostilities, despite the consequences of letting an invading nation retain seized territory.

"A scenario where China is increasingly aggressive, a Trump-like president is elected in 2024, and populist forces gain ground in Europe is a scenario where Russia could be a quasi-winner whatever happens on the ground in the next six months militarily," said Bruno Tertrais of the French Foundation for Strategic Research.

European challenges

Some 70,000 protesters rallied in Prague this month, accusing the Czech government of putting Ukraine's plight above its own citizens facing a spike in winter heating costs.

Such resentments could easily gain traction in other European nations, some of whom will soon be holding elections.

"At a time when other social fragilities are going to emerge, or re-emerge like the 'yellow vests' in France, it's going to put pressure on countries with leadership problems," said Emmanuel Dupuy of the IPSE European security think-tank.

He cited "France, where there's only a relative parliament majority; Germany, where the coalition is wobbly; and Spain and Britain."

In Italy, a populist far-right party suspected of being more disposed to working with Russia is well placed for elections on September 25.

Analysts warn the impact of tough European Union sanctions against Moscow will have a heavy toll on European firms excluded from the Russian market, dragging down economies and possibly pushing some into recession.

Soaring energy prices following Russia's clampdown on natural gas shipments to Europe will also hit millions this winter.

US resolve?

US President Joe Biden has provided Kyiv with billions of dollars in military equipment and financial aid, but a stagnant war in Ukraine could distract Washington.

"The Ukraine war is an irritant for the US… the US would be much happier if it could concentrate its effort in the Indo-Pacific," Charles Powell, director of the Spanish strategy think-tank Elcano, told a recent forum in Bucharest.

Washington is increasingly concerned with countering a more aggressive China in a region that is crucial for the globalized economy.

Some experts even warn of a "Thucydides Trap," a geopolitical theory in which confrontation between a declining power (the US) worried about a rising rival (China) becomes inevitable.

"There is a conflict coming. I don't know when, but it is less than five years away," US General Ben Hodges, former commander of US Army forces in Europe, told the Bucharest forum.

And the prospect that Donald Trump, who makes no secret of his admiration for Russian President Vladimir Putin and other authoritarian leaders, regains the White House in 2024 also cannot be excluded.

"What happens if the next president of the US decides that supporting Ukraine to the same extent is not in the US national interest?" Powell said.

Even November midterm elections, which could see Biden lose his left-wing majorities in the House and Senate amid skyrocketing inflation caused by the war, could jeopardize US support.

"All aid has to go through Congress, that's why the Biden administration is trying to push through as much aid as possible, in case it gets blocked by a less favorable Senate after the elections," said Maud Quessard, director of the French military academy's strategic research institute.