In the first century BC, Caesar and Octavian established the ancient Roman Empire, and China at this time was in the Western Han Dynasty, and the two most powerful civilizations in the East and the West entered their heyday almost at the same time. The difference is that the ancient Roman Empire went further and further on the road of expansion and reached its peak 200 years later, while the Western Han Dynasty began to decline until it was usurped by Wang Mang.
Many people in later generations attributed the decline of the Western Han Dynasty to the exhaustion of the Han Dynasty by the Han Dynasty, exhausting the national strength in order to fight against the Xiongnu, and the country became poorer and poorer. But the strange thing is that the ancient Romans never stopped using their armies, it can be said that this empire is based on continuous military expansion, so why didn't they fight more and more poorly like the Western Han Dynasty?
If you want to answer this question, you should say from the two aspects of cost and benefit, fighting a war is actually the same as doing business, the benefits exceed the cost, and the business that makes money is naturally getting bigger and bigger, if the reverse is reversed, the cost is higher than the income, and the business cannot be done.
From a cost point of view, the Western Han Dynasty's war against the Xiongnu required national mobilization, mobilizing grain, grass, taxes and manpower throughout the country. Every time the Han army went out to conquer Saiwai, it had to gather 500,000-100,000 cavalry and 200,000-500,000 infantry, and the number of civilians transporting grain and grass ordnance for the army would not be lower than the number of soldiers. In other words, the Han army went out to fight as few as 500,000 people and as many as 1 million.
Compared with the huge number of people, the long road is a bottomless pit, and the ancients have long made calculations, in order to transport 1 stone of grain to the front line a thousand miles away, at least 30 stones of grain must be eaten on the road. Starting from along the Great Wall, the Han army had to cross at least 2,000 miles of the Gobi to capture the Xiongnu royal court, and the grain and grass consumed in vain along the way could be imagined.
The ancient Romans were fortunate in this regard, there was a huge Mediterranean sea, and the cost and attrition of shipping was only a few tenths of that of land transportation. The Ancient Roman Empire was actually a Mediterranean coastal country, and once it was separated from the sea, the ancient Romans had no ability to expand too far inland.
If the ancient Romans were placed on the edge of the Mongolian desert, they would never have performed better than the Western Han Dynasty. In real history, when the ancient Romans entered the Germanic barbarian areas in the north and the Persian sphere of influence in the east, their combat effectiveness would decline sharply, and it would be difficult to even protect themselves.
After talking about costs, let's talk about benefits. The benefits of the Western Han Dynasty against the Xiongnu were close to zero, at best, it guaranteed its own safety, and occasionally grabbed some cattle, sheep and horses, and it was far from making up for the cost of the expedition. More importantly, the northern desert of Saibei was north of the 400 mm precipitation line, which was not suitable for the survival of farming peoples, and the Western Han Dynasty could not immigrate to the real side.
The ancient Roman Empire was completely different, and in the early days of expansion, many surrounding peoples were more civilized and richer than the ancient Romans, such as the Carthaginians. After conquering these peoples with swords, the ancient Romans completely appropriated the wealth they had accumulated for hundreds of years, and plundered hundreds of thousands of people as slaves, so that these ancient peoples were completely destroyed.
The expansion wars of the ancient Romans were like poor boys breaking into big cities, and the more territory they grabbed, the more wealth they had. And the Western Han Dynasty fought the Xiongnu, more like the self-cultivators planting trees on the edge of the desert, planting not enough, the desert will erode the fertile land, but if you plant too much, you can't take care of the fertile land, and the days are of course getting poorer and poorer.