The Trump administration opened a new military front April 6 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:
U.S. Special Operations personnel take cover to avoid flying debris as they prepare to board a UH-60 Black Hawk helicopter during a mission in Kunar province, Afghanistan, on Feb. 25, 2012. (Dept. of Defense photo by Petty Officer 2nd Class Clayton Weiss, U.S. Navy)
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.
An M2A2 Bradley in action during a mission in Iraq. (U.S. Air Force)
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.
3. Drone Wars
Predator drone strike.
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.
A quick reaction force with Special-Purpose Marine Air-Ground Task Force Crisis Response prepares to depart Naval Air Station Sigonella, Italy, in support of a military assisted departure from the U.S. Embassy in Tripoli, Libya, Saturday, July 26, 2014. (U.S. Marine Corps photo by 1st Lt. Maida Kalic)
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.
5. Islamic State Group in Iraq and Syria
U.S. Marines with Task Force Spartan, 26th Marine Expeditionary Unit (MEU), on Fire Base Bell, Iraq, fire an M777A2 Howitzer at an ISIS infiltration route March 18, 2016. (U.S. Marine Corps photo by Cpl. Andre Dakis)
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 IS. 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 IS since 2014. More recently, the U.S. has dispatched growing numbers of special operations forces to assist Kurdish and Arab forces fighting IS. Roughly 500 U.S. fighters are in Syria, plus additional, "temporary" forces that rotate through.
The guided-missile destroyer USS Porter (DDG 78) is one of the two warships to fire 59 BGM-109 Tomahawks at the Syrian airfield on April 6, 2017. (U.S. Navy photo)
Even while fighting IS in Syria, the U.S. has avoided wading into Syria's civil war by directly confronting Syrian President Bashar Assad — until now. On April 6, U.S. warships in the Mediterranean Sea launched some 60 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.