A YEAR after the deadly attack on the U.S. mission in Benghazi, Libya, President Obama’s pledge to bring the perpetrators to justice has yet to be fulfilled. Numerous people who participated in the assault have been identified, and some sealed indictments have been issued. But, as The Post’s Karen DeYoung reported, no one has been taken into custody, and authorities still haven’t clarified who plotted the attack and whether it was timed for Sept. 11 or carried out in response to anti-American protests that day in Cairo.

That’s an understandable cause of frustration for Republicans in Congress as well as some military and law enforcement officials, the New York Times reports. The fact that leading suspects in the attack, which killed ambassador J. Christopher Stevens and three others, operate openly in Benghazi, occasionally offering interviews to U.S. media, is particularly galling. Some wonder why the administration does not push the Libyan government harder to take action against the suspects or the Ansar al-Sharia militia, which joined the assault. Others say Mr. Obama should launch a unilateral U.S. raid, like that which killed Osama bin Laden.

In fact, there are good reasons for prudence. Setting aside the reality that Mr. Obama already has placed U.S. forces on alert for possible action in Syria, an American action in Libya could have a high political cost, even if it succeeded. The Libyan government and much of the population views the United States favorably because of its help in overthrowing dictator Moammar Gaddafi; a strike could squander that rare goodwill in an Arab state. It could also further destablize a moderate regime that already is struggling to keep the country’s economy functioning and complete the construction of a new democratic political system.

Libya’s government doesn’t act against Ansar al-Sharia or arrest terrorist suspects because it can’t. Two years after the overthrow of the Gaddafi regime, the country is still mostly controlled by a patchwork of militias that organized during the revolution and never disbanded. As The Post’s Kevin Sullivan recently reported, some are paid by and officially report to the government, but they don’t necessarily take its orders. For the last two months, militia members have taken control of key oil terminals, reducing exports to 15 percent of capacity.

What Libya needs from Washington is not a special forces raid but much more help in building a state. In a letter to Secretary of State John F. Kerry released this week, 28 experts, including former American diplomats, scholars and businessmen, urged a more active policy that would expand beyond seeking justice for the Benghazi attack. They urged U.S. technical support for the drafting of a new constitution that safeguards human rights, help in developing a long-term strategy to create an independent judiciary and training programs for security forces.

After helping to liberate Libya, the Obama administration and its European allies were too quick to walk away, leaving a shattered country to find its own way. If they wish to avoid another Arab state descending into chaos, they need to come back.