If Trump withdraws U.S. support, what next for European defense?

U.S. President Donald Trump's demand that Europe take responsibility for its own security has led to deep uncertainty about the future of the transatlantic alliance. It could potentially end decades of the so-called Pax Americana – relative peace in Europe guaranteed by the presence of U.S. troops.

Europe is now urgently looking at what options it has and what it all might cost.

More than a dozen European leaders plus Canada's Justin Trudeau traveled to Kyiv on Monday to mark the third anniversary of the conflict in Ukraine and reaffirm their military support even if the United States withdraws.

The UK and German leaders participated via videolink. Among those who didn't attend was Italy's Prime Minister Giorgia Meloni.

That same day France's President Emmanuel Macron met with President Donald Trump in the White House, assuring him Europe was prepared to pay its way.

"Europeans must do more for European security, its defense, and a fairer sharing of the security burden that your country has carried for several years," Macron said at a joint press conference.

Guarantee withdrawn

When NATO was formed in the aftermath of World War II, the U.S. guaranteed Europe's security, heralding a period of relative peace and prosperity. With the fall of the Berlin Wall and the collapse of the Soviet Union, European governments reaped a peace dividend, cutting military spending and using the money to fund social welfare programs.

Now defense spending is the new priority – but where will the money, in the hundreds of billions of dollars, come from? And what sort of painful decisions will European governments have to make between boosting defense and maintaining social welfare?

A new study says that in order to defend Ukraine and deter further threats to its sovereignty, Europe would need an additional 300,000 troops. And it's not just extra boots on the ground. 

According to the Bruegel and Kiel Institute study, new military hardware will be required including: 1,400 extra tanks, 2,000 more infantry vehicles and 700 additional artillery pieces. That's more combat power than currently exists in the French, German, Italian and British land forces combined. 

Emergency summit

To achieve all this rearmament would cost more than $260 billion extra a year. Next week the EU will hold an emergency summit on how to strengthen the bloc's defense – but whatever the way forward, it's going to cost. 

On Tuesday, UK Prime Minister Keir Starmer announced Britain's defense budget would increase to 2.5 percent of GDP by 2027, with foreign aid being cut.

"I can announce this government will begin the biggest sustained increase in defense spending since the end of the Cold War," Starmer told parliament, adding that to help pay for this he was cutting foreign aid from 0.5 percent of GDP to 0.3 percent.

Many European economies are already facing a cost-of-living crisis so raising taxes, increasing borrowing or cutting welfare programs to pay for defense could prove hard to sell to voters.