In the fifty years after World War II, the United States and western Europe generally maintained close diplomatic relations. Though they were never in total agreement, they usually worked together to promote international consensus under U.S. guidance. Over time, however, the growing power of the European Union and the new unilateral thrust of Washington’s foreign policy created strains in traditional transatlantic relations.

The growing gap between the United States and Europe had several causes. For one, the European Union was now the world’s largest trading block, challenging the predominance of the United States. For another, under presidents George W. Bush (r. 2001–2009) and Barack Obama (r. 2009– ), the United States often ignored international opinion in pursuit of its own interests. Citing the economic impact, Washington refused to ratify the Kyoto Protocol of 1997, which was intended to limit global warming and which had been agreed to by nearly two hundred countries. Nor did the United States join the International Criminal Court, a global tribunal meant to prosecute individuals accused of crimes against humanity, which nearly 140 states agreed to join.

A values gap between the United States and Europe contributed to cooler relations as well. Ever more secular Europeans had a hard time understanding the religiosity of many Americans. Relatively lax gun control laws and the use of capital punishment in the United States were viewed with dismay in Europe. Despite Obama’s health-care reforms — which provoked controversy among Americans — U.S. reluctance to establish a single-payer, state-funded program surprised Europeans, who saw their own such programs as highly advantageous.

Hardball geopolitical issues relating to NATO further widened the gap. The dissolution of the Communist Warsaw Pact left NATO without its Cold War adversaries. Yet NATO continued to expand, primarily in the territories in the former East Bloc. NATO’s expansion angered Russia’s leaders, particularly when President Bush moved to deploy missile defense systems in Poland and the Czech Republic in 2008. Even within the alliance there were tensions. By 2009, with twenty-eight member states, it was difficult to shape unanimous support for NATO actions. As the EU expanded, some argued that Europe should determine its own military and defense policy without U.S. or NATO guidance.

American-led wars in Afghanistan and Iraq, undertaken in response to the September 11 terrorist attacks against the United States, further strained U.S.-European relations. On the morning of September 11, 2001, passenger planes hijacked by terrorists destroyed the World Trade Center towers in New York City and crashed into the Pentagon. Perpetrated by the radical Islamist group al-Qaeda, the attacks took the lives of more than three thousand people from many countries and put the personal safety of ordinary citizens at the top of the West’s agenda.

Immediately after the September 11 attacks, the peoples and governments of Europe and the world joined Americans in heartfelt solidarity. Over time, however, tensions between Europe and the United States re-emerged and deepened markedly, particularly after President Bush declared a unilateral U.S. war on terror. The main acts in Bush’s war on terror were a U.S.-led war in Afghanistan, which started in 2001, and another in Iraq, which lasted from 2003 to 2011. Both succeeded in quickly bringing down dictatorial regimes. At the same time, they fomented anti-Western sentiment in the Muslim world and failed to stop regional violence driven by ethnic and religious differences.

The U.S. invasion of Iraq and subsequent events caused some European leaders, notably in France and Germany, to question the rationale for and indeed the very effectiveness of a “war” on terror. Military victory, even over rogue states, would hardly end terrorism because terrorist groups easily moved across national borders. Terrorism, they concluded, was better fought through police and intelligence measures.

American conduct of the war on terror also raised serious human rights concerns. The revelation of the harsh interrogation techniques used on prisoners held by American forces and abuse of prisoners in Iraq shocked many Europeans. U.S. willingness to engage in “extraordinary rendition” — secretly moving terrorism suspects to countries that allow coercive interrogation techniques — further caused concern.

The election of Barack Obama, America’s first African American president, in 2008, and his re-election in 2012, brought improvement to U.S.-European foreign relations. Upon election, President Obama announced that he would halt deployment of missiles in central Europe and reduce nuclear arms, easing tensions with Russia. He took U.S. troops out of Iraq in 2011, promised to withdraw U.S. combat troops from Afghanistan in 2014, and quietly shelved the language of the “war on terror.” In February 2013, the president’s call for a free-trade agreement with the European Union, which would end tariffs and regulatory barriers to trade, raised hopes for closer economic and political cooperation in the future. Despite these changes, many Europeans continued to find U.S. willingness to undertake unilateral military action disturbing.6 In the long run, though ties with the United States remained solid, European states increasingly responded independently to global affairs.