Barack Obama and David Cameron will tell their Nato allies to increase defence spending in a showdown at this week’s summit in Wales.
They are backed by British military chiefs who accused underspending members of freeloading.
The US president and the Prime Minister will call on European leaders to do more to defend the continent from its growing threats by meeting commitments to spend at least two per cent of GDP on defence.
Nato members at the two-day meeting in Newport will debate how to cope with crises in Ukraine and Iraq.
The White House said military spending was a “top priority”. But diplomats believe that the plea will be rejected by countries who complain they are still emerging from recession and refuse to be held to an “arbitrary” spending target.
Currently only four Nato members reach the two per cent target, including Britain and America. Britain, however, has refused to commit to the target after this parliament and defence chiefs fear the military budget could dip below the two per cent mark soon.
Former senior military commanders said last night that Nato allies must shoulder their share of the military burden and stop relying on others to prop them up.
Lord West, a former head of the Navy, said: “People in a number of countries have been willing to get a free ride and are not spending. If you look at Europe, it’s only France and the UK who meet the target.
“Nato has to realise that it’s no good having a broken force.
“Putin and other countries think they are talking the talk but they are not walking the walk. It’s just bluff.
“It does mean that he thinks they are not really serious about holding their own on the world stage.”
Lord Dannatt, a former head of the Army, said Europe’s failure to fund its forces led many Americans to question “why Europe cannot stand on its own security feet”.
He said: “The sad fact is that with the exception of a small number of European Nato member states — which include the UK and France principally — the vast majority of the armed forces of other European states lack real usable capability and their governments often lack the political will to fund their armed forces properly.”
Diplomatic sources said there was little hope of major European powers pledging big spending increases.
Any summit agreement will fall back on vague language of “aiming to” meet the two per cent target.
In pointed remarks last week, Mr Obama said the summit had to “make sure every country is contributing”.
Wolfgang Schäuble, the German finance minister, said earlier this year increasing defence spending from 1.3 per cent could create misunderstandings with Russia. The Canadian government has also baulked at the rise from its current level of one per cent.
Robert Menendez, the hawkish Democrat chair of the US Senate foreign relations committee, urged Mr Obama and the West to provide Ukraine with “defensive weapons”.
“Thousands of Russian troops are here with tanks, missiles, heavy artillery,” he told the US broadcasters CNN in Ukraine. Mr Putin had “sized up the West” and determined that tough sanctions and military aid would not be forthcoming.
However, Mr Obama and European leaders have ruled out any “military solution” to the crisis in Ukraine.
Meanwhile fighting continued in Ukraine with separatists firing on a border patrol ship in the Azov Sea.