MEXICO CITY (Reuters) - The spectacular fall from grace of Mexico’s previous armed forces supremo has raised awkward questions about the president’s reliance on the military to fight drug gangs and manage an increasing portfolio of vital civilian infrastructure. Thursday’s arrest of former Defense Minister Salvador Cienfuegos in the United States at the Los Angeles airport on drug trafficking charges sent shockwaves through the political establishment and embarrassed a once highly trusted institution. It threatens to sour government relations with the military, which since President Andres Manuel Lopez Obrador assumed power in December 2018 has been tasked not just with reducing violence, but also managing ports and even building an airport. “He has placed his entire political capital on making his political project work through the armed forces,” said Falko Ernst, a senior analyst with the International Crisis Group. “If he steers away from that, there’s no one else to turn to right now. He doesn’t have many other options left.” Lopez Obrador responded to the Cienfuegos arrest by vowing to root out military corruption - a promise which sits uneasily with the faith he previously put in Mexico’s generals. Now the same military leaders the president is counting on to pacify Mexico after years of gang violence may end up sidelined in top civilian security appointments, officials say. Before Cienfuegos’ detention, speculation was growing that Lopez Obrador would appoint a general to replace Security Minister Alfonso Durazo, who has flagged his intention to run for the governorship of the northern state of Sonora in 2021. “This is a game changer,” said a senior Mexican police official, who spoke on condition of anonymity. Lopez Obrador would have to “pay a high political price” to put a military figure in charge of the civilian body, the official added. Then there is the question of bad blood. Charging Cienfuegos, who ran the army from 2012-18, with the very crime he was meant to be rooting out risks shaking a key pillar of the Mexican state to its core - and incurring its wrath, another government official said. “The army won’t take this too well,” said the official. Lopez Obrador on Friday resoundingly backed Cienfuegos’ successor at the head of the army and his counterpart in the navy, saying he had personally vetted them and vouched for their honesty. Generals have been locked up on drugs charges before. But the arrest of the army’s one-time chieftain by a foreign power is a heavy blow to the prestige of an institution that has prided itself as the principal guarantor of stability in the country since the Mexican Revolution a century ago. In part this is because successive Mexican presidents, who can only serve a single six-year term, have opted to chop and change alternative, civilian police institutions rather than build on what their predecessors began, critics say. Yet that was often a reaction to the perceived corruption inside those bodies, as evinced by the December 2019 arrest of former Security Minister Genaro Garcia Luna by U.S. officials for allegedly taking bribes from the Sinaloa cartel. Garcia Luna denies the charges. Ironically, Lopez Obrador said the arrest of Cienfuegos flowed from that probe. NEGATIVE SPIRAL Lack of civilian alternatives to the military risks creating a negative long-term spiral, analysts say. Alejandro Hope, a former Mexican intelligence official, said if security is handed over to the military, there is no point in investing in the training of civilian commanders. “After a few years, there are no civilian commanders to turn to and there is no alternative but to leave the responsibility to the military,” he wrote in the El Universal newspaper. Once an outspoken critic of the military himself, Lopez Obrador has bolstered the armed forces’ economic interests by tasking the army to build a $3.2 billion airport for Mexico City. He has also ordered the navy to take over the running of key ports that Mexican cartels have infiltrated. In his political declarations, Lopez Obrador is often at pains to exalt the esprit de corps and probity of the army, despite ample past evidence of corruption. Some people close to the government were even privately celebrating Cienfuegos’ detention, saying it only confirmed their suspicions about rot in the military. “This breaks with the historic pact of impunity that protected the army,” said a senior member of the president’s ruling party, the National Regeneration Movement (MORENA). In a sign of the U.S. mistrust in Mexican officials, the government in Mexico City was not kept in the loop about details of Cienfuegos’ alleged crimes or the timing of the arrest. Officials expressed shock and amazement about the news. “There was no knowledge of it in security areas,” one senior government official told Reuters.