There is allegedly a Chinese saying that you can conquer a country on horseback, but you can't rule it on horseback. The same holds true for tanks. Eventually the tax collectors and the regulators have to get out of the tanks. Then they're vulnerable. The object is not to take on the Army. They're not the enemy. The object is to take on the real enemy.
Pretty close: The aphorism dates to the Yuan Dynasty occupatrion of China by Kublai Khan, grandson of Temujin, the Gengiz Khan.
However, if you do not particularly care if you rule a conquered country or city, you can deal with it in the manner of another of Temujin's grandsons, Hulagu Khan, who conquered and occupied Baghdad in 1258. After the Mongols spent a week of massacre, looting, rape, and general destruction. The Grand Library of Baghdad, containing countless precious historical documents and books on subjects ranging from medicine to astronomy, was destroyed. Survivors said that the waters of the Tigris ran black with ink from the enormous quantities of books flung into the river.
Citizens attempted to flee, but were intercepted by Mongol soldiers who killed with abandon. Martin Sicker writes that close to 90,000 people may have died. Other estimates go much higher. Wassaf claims the loss of life was several hundred thousand. Ian Frazier of The New Yorker says estimates of the death toll have ranged from 200,000 to a million.
The deposed Abbasid caliph was captured and forced to watch as his citizens were murdered and his treasury plundered. According to most accounts, the caliph was killed by trampling. The Mongols rolled the caliph up in a rug, and rode their horses over him, as they believed that the earth was offended if touched by royal blood. All but one of his sons were killed, and the sole surviving son was sent to Mongolia.
The death toll was so complete that Hulagu had to move his camp upwind of the city, due to the stench of decay from the ruined city. The destruction of Baghdad was to some extent a military tactic: it was supposed to convince other cities and rulers to surrender without a fight. Baghdad was a depopulated, ruined city for several centuries and only gradually recovered some of its former glory. "Iraq in 1258 was very different from present day Iraq. Its agriculture was supported by canal networks thousands of years old. Baghdad was one of the most brilliant intellectual centers in the world. The Mongol destruction of Baghdad was a psychological blow from which Islam never recovered. Already Islam was turning inward, becoming more suspicious of conflicts between faith and reason and more conservative. With the sack of Baghdad, the intellectual flowering of Islam was snuffed out. Imagining the Athens of Pericles and Aristotle obliterated by a nuclear weapon begins to suggest the enormity of the blow. The Mongols filled in the irrigation canals and left Iraq too depopulated to restore them." (Steven Dutch)
"They swept through the city like hungry falcons attacking a flight of doves, or like raging wolves attacking sheep, with loose reins and shameless faces, murdering and spreading terror...beds and cushions made of gold and encrusted with jewels were cut to pieces with knives and torn to shreds. Those hiding behind the veils of the great Harem were dragged...through the streets and alleys, each of them becoming a plaything...as the population died at the hands of the invaders." (Abdullah Wassaf as cited by David Morgan)
Causes for agricultural decline
Some historians believe that the Mongol invasion destroyed much of the irrigation infrastructure that had sustained Mesopotamia for many millennia. Canals were cut as a military tactic and never repaired. So many people died or fled that neither the labor nor the organization were sufficient to maintain the canal system. It broke down or silted up. This theory was advanced by historian Svatopluk Souček in his 2000 book, A History of Inner Asia and has been adopted by authors such as Steven Dutch.