The Trump administration opened a new military front Thursday when it ordered dozens of cruise missiles against a Syrian air base, adding to a growing list of recent U.S. military forays. A look at where the United States has fought in the 21st century:
After al-Qaida attacked the U.S. on Sept. 11, 2001, the U.S. led an invasion of Afghanistan that ousted the Taliban. Though the U.S. and NATO formally ended their combat mission in Afghanistan at the end of 2014, the war — now in its 16th year — drags on.
Some 8,400 American troops are deployed in Afghanistan, where they train the country's military and perform counterterrorism operations.
Under President George W. Bush, the U.S. invaded Iraq in 2003 and toppled Saddam Hussein. Bush's successor, President Barack Obama, pulled U.S. troops out of Iraq in 2011 after failing to reach an agreement with Baghdad to leave a residual U.S. force behind.
But the U.S. sent troops back three years later after the Islamic State group, a successor to al-Qaida in Iraq, seized Iraqi territory and sought to declare an Islamic caliphate.
Under Obama, the U.S. dramatically increased the use of unmanned aerial vehicles, also known as drones, to launch counterterrorism strikes without the need for a large U.S. military presence on the ground. The CIA and Defense Department have launched strikes in Pakistan, Yemen, Somalia and Libya, some of them covert.
Intense criticism from civil liberties advocates led Obama to create legal parameters for drone use that he hoped future presidents would respect. At least 117 civilians were killed from 2009 to 2016 by drone strikes outside of traditional warzones, the U.S. intelligence community has said. Other estimates place the toll higher.
The U.S. and European allies launched an air campaign in Libya in 2011, aiming to prevent atrocities by strongman Moammar Gadhafi against Arab Spring-inspired opponents. The bombing campaign toppled Gadhafi, but Libya slid into chaos and infighting. The Islamic State group later gained a foothold.
The U.S. has continued to carry out airstrikes in Libya that Washington says has diminished the number of IS extremists operating there.
ISIS IN IRAQ AND SYRIA
After IS captured a wide swath of Iraq and Syria in 2014, Obama announced the U.S. could target the group "wherever they are."
The U.S. started sending small numbers of military advisers to help Iraq's weakened military fight ISIS. The number has crept up to around 7,500 U.S. troops. IS has lost much of its former territory.
In Syria, the U.S. has conducted airstrikes against ISIS since 2014. More recently, the U.S. has dispatched growing numbers of special operations forces to assist Kurdish and Arab forces fighting ISIS. Roughly 500 U.S. fighters are in Syria, plus additional, "temporary" forces that rotate through.
Even while fighting ISIS in Syria, the U.S. has avoided wading into Syria's civil war by directly confronting Syrian President Bashar Assad — until now. On Thursday night, U.S. warships in the Mediterranean Sea launched 59 Tomahawk missiles at an air base in response to a chemical weapons attack blamed on Assad's forces.
The strikes mark the first direct U.S. attack on Syria's government, which has waged a six-year civil war against opposition groups. It also puts the U.S. into a de facto proxy battle with Russia's military, which is on the ground in Syria and has propped up Assad.