CARTAGENA, Colombia — A summit of 33 Western Hemisphere leaders opened Saturday with the United States and Canada standing firm, but alone, against everyone else’s insistence that Cuba join future summits.

Colombian President Juan Manuel Santos, a U.S. ally, opened the summit with a criticism of Cuba’s exclusion, calling it an unjustified anachronism of the Cold War.

He also urged a reconsideration of the war on narcotics that he said began a century ago, referring obliquely to growing suggestions that the hemisphere’s nations consider ending a prohibition of many drugs that has fed violence and crime.

The Sixth Summit of the Americas has also taken on a tabloid tinge with 12 U.S. Secret Service agents sent home for alleged misconduct.

President Obama has been clinging to a rejection of Cuban participation in the summits, which everyone but Canada deems unjust.

“This is the last Summit of the Americas,” said Bolivia’s foreign minister, David Choquehuanca, “unless Cuba is allowed to take part.”

The fate of the summit’s final declaration has been thrown into uncertainty as the foreign ministers of Venezuela, Argentina and Uruguay said Friday their presidents wouldn’t sign it unless the United States and Canada remove their veto of future Cuban participation.

Vigorous discussion is also expected on drug legalization, which the Obama administration opposes.

And Obama will be in the minority in his opposition to Argentina’s claim to the British-controlled Falkland Islands.

Obama may be able to charm the region’s leaders as he did in 2009 with a pledge of being an “equal partner,” but he will also have to prove the United States truly values their friendship and a stake in their growth.

“The United States should realize that its long-term strategic interests are not in Afghanistan or in Pakistan but in Latin America,” Santos said in a speech to business leaders at a parallel CEO summit Friday.

In large part, declining U.S. influence comes down to waning economic clout, as China gains on the United States as a top trading partner.