REPUBLICANS HAVE a potentially strong case to make against the Obama administration’s handling of Libya, as the latest political developments there underline. On Sunday, a disputed vote in parliament led to the swearing-in of a new prime minister — the sixth since former dictator Moammar Gaddafi was overthrown in 2011 with the help of U.S. and NATO air forces. The new leader, an Islamist from the city of Misurata, replaced pro-Western prime minister Ali Zeidan, who was driven out of the country this year after his government proved unable to stop a militia from filling a tanker with stolen oil.

From the safety of Europe, Mr. Zeidan conceded what was obvious all along: Libya’s post-Gaddafi government has no army and no way of establishing its authority over the hundreds of militias that sprang up in the vacuum that followed the revolution. Libya has fragmented into fiefdoms, its oil industry is virtually paralyzed, massive traffic in illegal weapons is supplying militants around the region and extremist groups such as Ansar al-Sharia, which participated in the Sept. 11, 2012, assault on the U.S. Consulate in Benghazi, are unchecked.

The Obama administration and its NATO allies bear responsibility for this mess because, having intervened to help rebels overthrow Gaddafi, they then swiftly exited without making a serious effort to help Libyans establish security and build a new political order. Congress might usefully probe why the administration allowed a country in which it initiated military operations to slide into chaos.