An obvious consideration is end strength, but American troops bring significantly more combat power than an equivalent number of European counterparts and can’t be replaced on a one-to-one basis, according to a report by think tank Bruegel and the Kiel Institute for the World Economy.

Tallying such requirements isn’t just an academic exercise anymore. President Donald Trump’s efforts to improve relations with Russia and reach a deal to end Vladimir Putin’s war on Ukraine have prompted European allies to confront the unthinkable: the effective end of NATO after almost 80 years.

The report from Bruegel and Kiel pointed out that Russia’s military has become much larger, more experienced, and better equipped since it invaded Ukraine three years ago, launching the bloodiest conflict in Europe since World War II.

“A Russian attack on a European Union country is thus conceivable,” it warned, citing recent intelligence assessments from NATO and various other countries.

In fact, Denmark is boosting defense spending 70% after its intelligence service said in a report this month that Russia could launch a local war against a bordering country within six months, a regional war in the Baltics within two years, and a large-scale attack on Europe within five years if the U.S. does not get involved.

In the event of another Russian attack on Europe, war planners have estimated that the 100,000 U.S. troops already stationed in Europe would be quickly joined by up to 200,000 U.S. reinforcements. But without them, the calculation is different for Europe.

“The combat power of 300,000 U.S. troops is substantially greater than the equivalent number of European troops distributed over 29 national armies,” the think tank said. “U.S. troops would come in large, cohesive, corps-sized units with a unified command and control tighter even than NATO joint command. Furthermore, U.S. troops are backed by the full might of American strategic enablers, including strategic aviation and space assets, which European militaries lack.”

To close the gap in combat power, Europe would have to boost end strength “significantly” by more than 300,000 or rapidly enhance military coordination, according to the report.

Europe also needs more weapons to fight off a Russian attack as well as transport, communications, and intelligence capabilities.

Of course, increasing personnel and equipment at scale will cost a lot, and the report estimated Europe must spend an additional 250 billion euros each year, or about 3.5% of GDP.

That money would have to be raised through debt offerings in the short term but would need a longer-term funding mechanism. The report said 125 billion euros could be raised annually for the next five years at the EU level, as individual EU member states increase their non-debt-funded share of spending.

Markets have anticipated a surge in European defense outlays, helping defense contractors like BAE Systems, Thales, Leonardo, and Saab soar since Trump was elected, outperforming U.S. giants like Lockheed Martin and Northrop Grumman.

But massive fiscal support could be tricky as Germany has a “debt-brake” rule that limits borrowing, while France is already under pressure from the bond market for its bulging deficits and high debt-to-GDP ratio.

Philippe Legrain, a former economic advisor to the president of the European Commission, suggested European governments could borrow collectively for a one-off investment in defense, citing the EU’s COVID response fund as an example.

In his Project Syndicate op-ed on Wednesday, Legrain also floated the idea of borrowing from the European Investment Bank, which already finances projects that have both civilian and military uses.

“However it is financed, Europe needs to rearm now,” he added. “Upping defense spending to avert Ukraine’s defeat and deter broader Russian aggression is much less costly than fighting an all-out war.”