Australia and New Zealand have integrated their own economies via a free trade agreement. While political and cultural ties with North America and Europe remain strong, economic reform and commodities trade with the booming economies of Asia have set the South Pacific nations on a new economic trajectory with Australia largely avoiding a downturn in the Financial crisis of 2007–2008 which unleashed severe economic loss through North America and Western Europe. Today Canada remains part of the Commonwealth, and relations between French and English Canada have continued to present problems. A referendum was held in Quebec, however, in 1980, in which Quebecers voted to remain part of Canada. In 1990, the white-minority government of the Republic of South Africa, led by F. W. de Klerk, began negotiations to dismantle the system of apartheid. South Africa held its first universal elections in 1994, which the African National Congress (ANC), led by Nelson Mandela, won by an overwhelming majority. The country has since rejoined the Commonwealth of Nations. Since 1991, the United States has been regarded as the world's only superpower. Politically, the United States is dominated by the Republican and Democratic parties. Presidents of the United States between 1980 and 2006 have been Ronald Reagan, George H. W. Bush, Bill Clinton, and George W. Bush. Since 1980, Americans have become far more optimistic about their country than they were in the 1970s. Since the 1960s, a large number of immigrants have been coming into the U.S., mostly from Asia and Latin America, with the largest single group being Mexicans. Large numbers from those areas have also been coming illegally, and the solution to this problem has produced much debate in the U.S. On 11 September 2001, the United States suffered the worst terrorist attack in its history. Four planes were hijacked by Islamic extremists and crashed into the World Trade Center, the Pentagon, and a field in Pennsylvania. The late-2000s financial crisis, considered by many economists to be the worst financial crisis since the Great Depression of the 1930s, was triggered by a liquidity shortfall in the United States banking system, and has resulted in the collapse of large financial institutions, the bailout of banks by national governments, and downturns in stock markets throughout much of the West. The United States and Britain faced serious downturn, while Portugal, Greece, Ireland and Iceland faced major debt crises. Almost uniquely among Western nations,