President George W. Bush said Tuesday that the 12-year-old trade agreement between the United States, Canada, and Mexico has aided the economies of all three countries and that cooperation on border security will keep commerce flowing. Bush praised the benefits of the North American Free Trade Agreement (NAFTA) as he concluded a summit in Montebello, Quebec with Canadian Prime Minister Stephen Harper and Mexican President Felipe Calderon. “NAFTA has yielded prosperity,” Bush said. “The question we are faced with is, how do we continue to enhance prosperity?” The summit focused on how the three countries can prepare better for natural and security disasters and keep commerce flowing as the U.S. tightens controls on its borders. Harper said the free flow of trade is no threat to national sovereignty. The three North American neighbors are “independent and interdependent,” he said. Asked about turmoil in financial markets, Bush and Harper said the fundamental elements of their countries' economies remain strong. “Is there enough liquidity in our system?” Bush asked. “The answer is ‘yes'.”