The Battle of Hue City during the Vietnam War still weighs heavily on military planners and strategists. So much so that for quite a while, both the Marines and Army failed to do anything substantial to prepare for large-scale urban combat — other than to avoid it whenever possible.

But both services are now updating their urban operations doctrine for the first time in years, and their leaders, Army Chief of Staff Gen. Mark Milley and Marine Corps Commandant Gen. Robert Neller, have publicly pushed for more urban-focused efforts.

The Chinese military strategist Sun Tzu foresaw the issues that are involved in city fighting. More than 2,500 years before the Battle of Hue City, he provided this maxim about the dangers of urban warfare.