At the end of March 1879, the Argentine forces that had suffered a crushing defeat in Santa Fe were severely weakened and could no longer cope with the threat of the Han Wehrmacht on three fronts at the same time.

After the loss of Santa Fe, the only two fortresses left in front of Buenos Aires were Rosario and Paraná, one large and one small.

The city of Paraná is a small coastal city located in the north of the middle section of the Paraná River, and in terms of strategic position and defensive capability, the city of Paraná, with a population of only more than 10,000, cannot be compared with Rosario, which has a population of more than 100,000.

As the most important strategic point to protect the security of Buenos Aires, Rosario's defense force quickly increased to 15,000 after the Battle of Santa Fe, and in order to resist the advance of the Han army to the greatest extent, Mitre sent an additional 5,000 young laborers to assist the Rosario defenders in building fortifications.

Eight days after the end of the Santa Fe War, Li Mingyuan led 140,000 Wehrmacht troops to spare the already definite city of Paraná and march south along the Paraná River to Rosario.

On the morning of 25 March, the main forces of the Wehrmacht launched a fierce attack on the defenders of Rosario from three directions: north, east and west.

At noon on the 28th, after three days of continuous offensive, the Wehrmacht succeeded in occupying the four lines of defense set up by the Argentine troops outside the city, and the spearhead of the three attacking forces was all directed at the core area of the fortress of Rosario.

It stands to reason that the garrison and fortifications of the fortress of Rosario are not small than that of Santa Fe, but what frightened and puzzled the generals of the defenders of Rosario was that under the blows of the Wehrmacht, Rosario's outer defense line held for a shorter time than that of Santa Fe.

The number and number of Wehrmacht units used for the front-line offensive have not changed much, and the military strength of the Argentine troops on the perimeter defense line is not inferior to that of the Santa Fe defenders, and the reason why the Wehrmacht was able to take the outer defense line of Rosario in a shorter time under the condition that the strength of the offensive and defensive sides has not changed much, is because more than a week has passed, the continuous heavy rain weather has finally stopped, and the transportation difficulties that plagued the logistics and supply of the Wehrmacht have been alleviated to a great extent. F-FDTL units at the front were able to receive improved new weapons such as mortars and grenades from the rear.

Mortars and grenades are weapons used in actual combat in the early days of the Wehrmacht, but because of the simplicity and lack of industrial technology, the mortars originally produced not only faced unfavorable factors such as short range, low accuracy, and difficult to aim and shoot, but also most importantly, because of the low nature of the steel raw materials used as mortar guns, resulting in the short service life of the mortars produced, and it is easy to be in danger of exploding due to improper operation and poor maintenance.

The improvement of mortar manufacturing technology relied on the support of steel technology, so it was not until the outbreak of the Argentine war that Hafield discovered the method of smelting manganese steel, and many of the problems that limited the development of mortars were quickly solved.

After using manganese as an additive to solve the hardening problem of steel, Hafield and others quickly carried out the improvement of mortar performance, and soon made substantial progress, while those mortars transported by logistics troops were the results of Hafield and others.

In the battle of attacking the outer positions of Rosario, it was precisely because of the use of new mortars that the Argentine fire support points were destroyed before they could play their due role, thereby speeding up the loss of the outer front of Rosario and providing favorable conditions for the Wehrmacht to quickly solve the peripheral battles.

At 7 o'clock in the morning of April 1, the Wehrmacht concentrated more than 20,000 troops of the 1st and 3rd Divisions to attack the northern port of Rosario.

At 9 o'clock in the morning, the 1st Regiment of the 3rd Division occupied the Argentine position in the estuary area, and a few minutes later, a group of more than 700 Argentine troops arrived at the estuary position and immediately launched a counterattack against the 3rd Regiment.

The roar of artillery fire, the screams of soldiers, mutilated or bleeding corpses piled up in layers in the cramped space.

The battle for the estuary was intense from the outset, with the positions occupied by the 3rd Regiment captured by the Argentines four times, and then the remnants of the 3rd Regiment, supported by additional troops, launched another counterattack against the Argentines and took the positions from the opposing side.

The traditional positional offensive and defensive battles turned into bloody tug-of-wars, and the Wehrmacht and Argentine defensive forces continued to throw their vital forces into the battle for the estuary position, until 12:10 p.m., when the 1st and 2nd Divisions crushed all the counterattacking Argentine troops at the cost of 800 casualties, and the battle for the estuary position was declared over.

At 2:40 p.m., Li Mingyuan ordered the 2nd Division and the 1st Mixed Brigade to take over the troops of the 1st and 3rd Divisions, which had been fighting continuously, and continue the reactionary attack on the remnants of the enemy in the northern port of Rosario, and the battle lasted until 4:20 p.m., when all the northern ports of Rosario fell into the hands of the Wehrmacht, and the last Argentine unit of more than 3,000 people left a large number of wounded and fled back to the city of Rosario, while the Wehrmacht completed the comprehensive siege of the Argentine defenders.

In the following three days, the Wehrmacht, under the command of Li Mingyuan, on the one hand, put on a posture of all-out siege of the city, and on the other hand, sent two troops to the west and east of Rosario to ambush the Argentine reinforcements in the direction of Córdoba and Buenos Aires.

After waiting for three days without waiting for the prey that had thrown himself into the net, Li Mingyuan immediately gave up the strategic plan of encircling the point and sending reinforcements, and instead launched a final attack on Rosario.

At 3 p.m. on April 5, the 155-mm heavy artillery regiment blasted a gap of seven or eight meters wide in the western wall of Rosario, and then the troops of the 1st Regiment of the 3rd Division entered the city along the gap under the leadership of the officer, and at 3:30 p.m., the Wehrmacht repelled the Argentine troops in the blockade, and the main force of the troops began to enter the city, and on April 7, after two days of fierce street fighting, the Wehrmacht initially cleared the enemy forces in the city, and a small number of routs hidden among the Argentine whites could no longer pose a threat to the Wehrmacht, so at 2:40 p.m., Li Mingyuan announced the end of the Battle of Rosario, and the Wehrmacht began preparations for the last large-scale battle of the Argentine War.