The weather in Hanjiang River lasts for 30 days.

One of the reasons for the rapid collapse of Nanming was that Zheng Zhilong's navy was unable to defend the Yangtze River defense line. Nanjing is located at the corner of the Yangtze River, and its terrain is very special. The Yangtze River forms a 90-degree right angle with the north and west of Nanjing, while the east and south are low hills and plains covering thousands of square kilometers. The Yangtze River in the north and west is very wide in Nanjing, with a width of 1 0,500m, i.e. 1.5km, a water depth of 15m to 30m, a maximum water depth of 70m and a flow rate of 3m per second. Once the war breaks out, the Nanjing regime must control the control of the Yangtze River if it wants to protect itself. If the Nanjing regime can't control the Yangtze River, it will be tantamount to handing over its north gate to the enemy, and Nanjing will never be able to keep it.

South and east of Nanjing are low plains, hills and hills. Draw a circle with Nanjing as the center, and there is no danger in the radius of 150 km south of the Yangtze River. The 100 and 200 kilometers from Nanjing to Zhenjiang, Jiangyin and Wuxi in the east are all plain areas with only a few rivers and lakes. The topography of Lishui, Liyang, Maanshan and Wuhu in the south is similar to that in the east, and there is no terrain suitable for defense within 100200 km. The enemies of Nanjing since ancient times have basically come from two directions, one is the north, the other is the west and the other is the south. Among them, it is not easy to succeed if you simply invade Nanjing from the north. Why? Mainly because there is a natural obstacle in the north of Nanjing, the Yangtze River. Nanjing Yangtze River 1.5km wide, so it naturally took longer to use wooden sailboats in ancient times. In stormy weather, the river surface would be closed. Many dynasties in history took Nanjing as their capital, but few really killed the enemy from the north! If you only attack Nanjing from the north, it is actually useless to attack one side! In history, there was a precedent of successfully defending a super-strong enemy near Nanjing, and this enemy was the powerful Jin Army. At that time, Wan Yanliang led 600,000 Jin Guo troops south, trying to destroy the Southern Song Dynasty. All the way south along the coast, he was defeated by the Southern Song Navy along the coast of Shandong, and almost all the troops were wiped out. Seeing that it was impossible to attack Hangzhou, Zhejiang Province from the sea, Wan Yanliang decided to attack by land and inland rivers, so he had to cross the natural barrier Yangtze River, which was defended by the Southern Song Dynasty. At that time, Yan Hongliang was too underestimating his enemy and chose Maanshan to cross the river near Nanjing. As a result, the most powerful water army in the Southern Song Dynasty gathered near Nanjing, and under the leadership of Yu, a scholar in the Southern Song Dynasty, it won a great victory in the battle of quarrying. After the crushing defeat in the battle of quarrying, Wan Yanliang did not give up, and immediately moved to Yangzhou to try to cross the river here, but was defeated by the Southern Song navy approaching Shuizhai. In a rage, Wan Yanliang ordered all the officers of Gao Jun to be beheaded three days later, and at the same time ordered the soldiers to escape and kill their comrades.

Xu Jin's officers and soldiers had enough and immediately launched a rebellion. Yan Hongliang was shot dead by random arrows, and his ambitious southward invasion was a complete failure. Why is it not easy to attack from the north, even with an army of 600 thousand like Yan Yanliang? Because the Yangtze River is in the north of Nanjing, if you want to occupy Nanjing from the north, you must annihilate the water army of the Nanjing regime. Unfortunately, it's hard to say. Since Nanjing is the capital of this regime, and the Yangtze River is a few hundred meters away from the gate of Nanjing, this regime will definitely attach great importance to the construction of the water army, no matter how retarded it is. There are not many lakes and rivers in the north, and the enemy water army from the north cannot be very strong. And even with a strong water army, it is difficult to reach the Yangtze River. Take Wan Yanliang as an example. In fact, he also had a water army in the north, but he was annihilated at sea when he went by sea, and because the water margin was dry season, big ships couldn't cross it, so his main water army couldn't reach the Yangtze River. Although Wan Yanliang, the king of Hailing, has elite Jurchen soldiers and horses, these troops are of no help in breaking through the Yangtze River. The water army that Jin Jun can fight in the Yangtze River, no matter in quantity or quality, is far from the opponent of the water army in the Southern Song Dynasty. Xu Jin's naval forces fighting in the Yangtze River are all gondolas, which are slow and have poor ability to resist wind and waves, and the equipped water warfare weapons are also quite backward. In the Southern Song Dynasty, the navy owned a sturdy dolphin boat (whale boat) which was the most suitable for water combat in the world at that time. It is large, mobile and fast. Not only is it equipped with a large number of naval weapons, but it is also easy to sink Jin Jun ships. At the same time, the Southern Song Dynasty also used the most advanced water warfare weapon at that time, that is, the catapult weapon that fired gunpowder bombs. In the previous naval battle, the gunpowder weapon rocket was used, which was the most advanced naval weapon at that time. If we can't defeat the water army of Nanjing regime, the enemy from the north can't attack Nanjing.

Therefore, it is rare to invade Nanjing from the north alone in history, and most of them were killed from the west and south along the Yangtze River. Although it is not easy to cross the river near Nanjing, it is relatively easy to cross the river in the middle reaches of the Yangtze River in Hubei. The current in this section is slow, there are many beaches and landing points, so it is difficult for the defender to cover everything, which is beneficial to the attacker. In addition, Hubei is close to Henan Province, and the terrain of the Central Plains plain, the northern cavalry can break through the defense of the Central Plains and reach the Yangtze River in a few days. Therefore, in ancient times, most northern invaders crossed the river from Hubei, and then sailed eastward and northward along the Yangtze River or marched simultaneously by land and water, attacking Nanjing from the west and south, crushing the defenders in the city. Then the main battlefield is actually not in Nanjing, but in Hubei. If the battle of Hubei can be won, there will be no danger in Nanjing. If the battle of Hubei fails, Nanjing's special terrain determines that it cannot resist the enemy from west to southwest, and failure is inevitable.

There are many such examples in history. For example, in the Southern Song Dynasty, the Mongols attacked Jingxiang, the Qing army attacked Daming, defeated Minghe peasant army in Hubei, and there were several wars in Hubei during the Three Kingdoms period. During the Three Kingdoms period, Cao Cao led hundreds of thousands of victorious troops south to destroy Dongwu, and Zhou Yu led 50,000 chosen men to fight Cao Jun in Chibi City, Hubei Province. Although Cao Jun had powerful infantry and cavalry, the water army was no match for Wu Jun, and eventually he was defeated. The water army was wiped out and the army suffered heavy losses. This is also the famous Battle of Red Cliffs. As a result, it is impossible for Cao Wei to destroy Dongwu for decades until it dies! The counterexample is that the main force of the Jin army, which successfully annexed Shu, dispatched six armies to destroy it. At that time, the Jinjun Water Army entered Nanjing mainly from Hubei and Anhui along the river. Wu Dong's navy fought a decisive battle with him in Hubei and Anhui, but he was defeated one after another. The iron rope for blocking the river and the iron cone at the bottom of the river did not work, and he completely lost control of the river. Then the water army of 8 Jin Army went straight to Nanjing. Although Sun Hao, the ruler of Wu, still has some troops, he can't control the military in Nanjing.

Seeing that the tide was gone, he surrendered immediately, ending the glorious Three Kingdoms era. Later, just like Jin Jun, the Taiping Army first captured Wuhan in Hubei from Hunan in the south, and then hundreds of thousands of soldiers and civilians marched eastward along the land and water, and captured Nanjing, where the Governor's Office of the Two Rivers in the Qing Dynasty was located, and thus established its capital here in 13. So it is not easy to attack Nanjing from the north, because it is more difficult to break the water army of the Nanjing government here. Then it is relatively easy to capture Nanjing from west to south along the Yangtze River, because the decisive battle is actually not in Nanjing at this time, but in Anhui, Hubei. Once the invading enemy can occupy the river control in Hubei, it will be difficult for Nanjing to hold it, because there is only the Yangtze River near Nanjing as a natural danger. If the enemy controls the Yangtze River, Nanjing will have no danger to defend.

There are few precedents for attacking Nanjing from the east in history, because the sea is more than 200 kilometers east of Nanjing, and there are naturally no enemies in ancient times. However, once there is such an attack, Nanjing will be difficult to hold. Because attacking Nanjing from the east, Nanjing is the least defenseless militarily. If we attack Nanjing from the north, west and south, the Yangtze River will become a natural disaster in Nanjing, and we must defeat Nanjing's water army to win. If the enemy attacks Nanjing from the east, it is naturally best for the invaders to have a strong water army. If not, it doesn't matter. You can even ignore the water army and attack completely by land. At this time, for defending Nanjing, the role of the Yangtze River is just the opposite. It is no longer a natural barrier, but the biggest obstacle for Nanjing defenders to retreat and transport reinforcements. Because the terrain near Nanjing is not suitable for defense, the defenders will be in a very unfavorable position. If we want to hold Nanjing, we must concentrate our forces stronger than the enemy for a decisive battle on land, and the decisive battle site can only be chosen at least 50 kilometers away from Nanjing, because the terrain near Nanjing is even more unsuitable for defense. More than ten kilometers outside Nanjing, there are only a few hillsides or hills except the Purple Mountain. This terrain is meaningless to the defenders, because the mountain is too short for the enemy cavalry to rush up. These terrains are meaningless to the defender, but they are different to the attacker.

Because the terrain of Nanjing is generally much lower than that outside the city, once the attacking enemies occupy these hills and hills, they can attack the city from these highlands, forming the best firepower point for long-range attack in the city. Therefore, from the general military point of view, if the peripheral decisive battle fails, Nanjing must abandon its defense and retreat immediately. There are also some successful examples in history of capturing Nanjing from the east, but most of them are recent. The inner wall of Nanjing faces Zhongshan in the east, with stones in the west, Qinhuai in the south and Houhu in the north, with a circumference of 35.267 kilometers. The existing basically intact Ming City Wall is 23.399 kilometers long, which is the longest existing ancient city wall in the world, one of the few well-preserved ancient city walls in China and the largest existing ancient building in Nanjing. This wall is an absolute killer in the era of cold weapons, and it is difficult for any army to capture it. Later, the Qing army attacked Nanjing, and the Nanjing garrison commander witnessed the Yangzhou massacre and dared not resist the surrender of Kaesong, otherwise it would be a bloody battle to the end. People of insight throughout the ages are well aware of the terrain of Nanjing. Therefore, Nanjing was not the capital of the Southern Song Dynasty, because the Jin Army was too strong at that time, and the Southern Song Dynasty fled the Yangtze River, which dealt a great blow to its combat effectiveness. Therefore, Song Gaozong and Zhao Gou are afraid that Nanjing can't keep it, while Hangzhou, which is far away from the Yangtze River, is much safer and its capital is Zhejiang. Zhu Yuanzhang's capital is Nanjing, and he also knows that Nanjing's defense ability is weak. In addition to establishing a navy to control the river regime, the Ming army also spent 2 1 year and huge sums of money to build the Nanjing city wall, which is more than 30 kilometers long. Nanjing City Wall is very strong, second only to the Angu City Wall in Xi. This 20-meter-high, 15-meter-long wall was extremely difficult to break in ancient times. Unfortunately, it has not been able to defend for a long time since it entered the era of gunpowder. At that time, the Taiping Army buried thousands of kilograms of black powder underground, blew down a section of the wall of Fengyimen in Nanjing, and finally rushed into the city. Therefore, the key to Nanming's sustained resistance lies in whether the Yangtze River Navy can control the river forces.