Battle of New Orleans

The capture of New Orleans (April 25 – May 1, 1862) during the American Civil War was a turning point in the war, which precipitated the capture of the Mississippi River. Having fought past Forts Jackson and St. Philip, the Union was unopposed in its capture of the city itself, which was spared the destruction suffered by many other Southern cities.
Many residents resented the controversial and confrontational administration of the city by its U.S. Army military governor, who caused lasting resentment. This capture of the largest Confederate city was a major turning point and an event of international importance.
Once this defense was breached, there remained to face Union troops and warships only three thousand militiamen with sundry military supplies and armed with shotguns. The city itself was a poor position to defend against a hostile fleet. With high water outside the levees, Union ships were elevated above the city and able to fire down into the streets and buildings below. Besides the ever-present danger of weather-caused breaks in the levees, now an even greater threat to New Orleans was the ability of the Union military to cause a break in a major levee that would lead to flooding most of the city, possibly destroying it within a day.
https://en.wikipedia.org/wiki/Capture_of_New_Orleans