[Explanation]: Back to water: Back to water, indicating that there is no way back. It is a metaphor for fighting to the death with the enemy.

【Idiom Story】:

This idiom comes from the Historical Records. The Biography of the Marquis of Huaiyin", the letter is to make ten thousand people go first, out, and turn their backs on Chen. ... The army is fighting to the death and cannot be defeated.

Han Xin, a native of Huaiyin (now southwest of Qingjiang, Jiangsu). He was a general under Liu Bang, the king of Han. In order to defeat Xiang Yu and seize the world, he made a plan for Liu Bang, first captured Guanzhong, then crossed the Yellow River eastward, defeated and captured Wei Wangbao, who betrayed Liu Bang and obeyed Xiang Yu's orders, and then attacked Zhao Wangxi to the east.

Han Xin's troops had to pass through an extremely narrow mountain pass, called Jingchengkou. Li Zuojun, a strategist under King Zhao, advocated blocking the Jingcheng Pass on the one hand, and sending troops to cut off the Han army's heavy grain and grass on the other hand, and Han Xin's expeditionary force would definitely be defeated without backup; But the general Chen Yu did not listen, relying on the superiority of troops, insisted on fighting the Han army head-on.

Han Xin was very happy to learn about this situation. He ordered his troops to camp thirty miles from Jingcheng, and in the middle of the night, let the soldiers eat some snacks and tell them that they would have a full meal after winning the battle. Subsequently, he sent 2,000 light cavalry to advance covertly from the path, and asked them to quickly rush into the camp of the Zhao army after the Zhao army left the camp, and change the banner of the Han army; He also sent 10,000 troops to deliberately line up their formations against the river to lure the Zhao army.

At dawn, Han Xin led his army to attack, and the two sides fought fiercely. After a while, the Han army pretended to be defeated and returned to the waterside position, and the Zhao army all left the camp and came to pursue. At this time, Han Xin ordered the main force to attack, and the soldiers who were in formation on their backs also turned around and pounced on the enemy because they had no way to retreat. The Zhao army could not win, and was about to return to the camp, when suddenly the camp was full of Han army flags, so they scattered and fled. The Han army took advantage of the victory to pursue and fought a great victory.

When celebrating the victory, the generals asked Han Xin: In the art of war, it is said that the formation can be backed by the mountain, and the front can be near the water, and now you let us line up with our backs to the water, and also say that we will defeat the Zhao army and eat a full meal, we didn't believe it at the time, but we actually won, what kind of strategy is this?

Han Xin smiled and said: This is also in the art of war, but you haven't noticed it. Doesn't the art of war say 'fall into the place of death and then live, and put the place of death and then survive'? If there is a place to retreat, and the soldiers have fled, how can they be desperate!

This story evolved into the idiom "Last Fight", which is mostly used in military operations, but can also be used in operations that are more decisive than those of a decisive nature.