The Chinese expression 强弩之末 literally means a spent arrow from a powerful crossbow.
During the early years of the Western Han Dynasty (206 BC - AD 24) the nomadic Huns made regular incursions across the northern border, plundering villages and killing many innocent people. Liu Bang, the first emperor of the Western Han sent thousands of troops to repel the Huns, but this failed. Finally, in 200 BC, Liu personally lead an army to the northern border to attack the enemy. However, the royal army was too weak and the Huns took them to the cleaners. Liu and his army were forced to retreat to the capital with their tail between its legs.
During the next 50 years, the battle with the nomadic Huns continued without respite, bringing great misery to the people and leaving them down in the dumps. It was not until the reign of Emperor Wu that the nation was again sufficiently prosperous and powerful to entertain the possibility of another pre-emptive attack on the Huns.
Emperor Wu's plan to send the entire royal army thousands of miles to the northern border was supported by most of his court officials. However, one outspoken official, Han An'guo, went against the flow. He said:
Today, we are quite strong. But it is widely known that after travelling a long distance, a strong gust of wind will fade into a tiny breath of air that cannot ruffle the feathers of a bird; and a spent arrow, though shot from a powerful crossbow, will not pierce a thin piece of silk. The same is true of armies. After our troops travel thousands of miles to the northern border, they will be exhausted and will be crushed by the Huns.
Emperor Wu paid no heed to Han's advice. Unfortunately for the Western Han Dynasty, the advice turned out to be accurate.
Today Chinese people use the expression 强弩之末 to describe something or someone that has, due to the passage of time or distance, become less powerful. The English call such persons or things a spent force.