CHILPANCINGO, MEXICO - NOVEMBER 14: Family members of 43 kidnapped students from the Ayotzinapa normal school line up for a march on November 14, 2014 in Chilpancingo, Mexico.

FOR DECADES, Mexico has been characterized as mired in corruption, and the stain of lawlessness has indeed run deep. But recent events have underscored anew how Mexico — despite genuine signs of economic and political progress — remains a state lacking the rule of law. A new low point is the disappearance of dozens of students from a teachers college, and much will depend on whether the government responds effectively.

Conflict among drug cartels dominated the six-year term of President Felipe Calderón, who threw the military into the battle at great cost but did not uproot the cartels. His successor, Enrique Peña Nieto, came to office in December 2012 with a different emphasis, seeking to kick-start a series of bold economic reforms. Mr. Peña Nieto rightly won admiration for his determination to open Mexico’s energy sector to competition, to tackle the troubled education system and to limit monopolization in telecommunications and other sectors. His first two years in power have been promising.

But the scourge of violence has risen again in a way that cannot be ignored and that imperils the reform agenda. Late one September night, in the town of Iguala in the poor, rural state of Guerrero, buses carrying the students were attacked by police. One student and six other people were killed. Forty-three students were captured and turned over to an organized-crime gang, Guerreros Unidos. They are feared dead. According to the Mexican authorities, the mayor of the town ordered the violent onslaught. The mayor, now detained, reportedly ordered the attack out of fear the students were going to interrupt a speech by his wife.

Protests have erupted across the country. Mr. Peña Nieto’s government did not respond with alacrity. The president was silent for days, and then his attorney general, exhausted, carelessly dismissed repetitive questions about the case at a news conference, saying he was “tired” or “fed up.” Those words have now become the signature slogan of the demonstrators; people are fed up with business as usual. Mr. Peña Nieto went ahead with plans to travel to Asia while the country was in an uproar, which did not help matters.

Mr. Peña Nieto cannot dodge this crisis. He must respond to public revulsion over the missing 43 students by making sure that a complete and transparent investigation is carried out, not only into the perpetrators but also into structural and institutional ills that have brought Mexico to this point. He can speed up implementation of important changes in criminal justice procedures, which has been slow. He must find a way to harness the public fury and use it to begin what will surely be a difficult struggle to firmly establish rule of law. The future of Mr. Peña Nieto’s economic reforms — and his own career — are at stake.