Discuss military tactics on paper
This chengyu refers to empty theoretical discussion of an issue that won't actually serve to resolve that issue. It also refers to idle discussion that can't become reality
Negative
有些学者只会纸上谈兵，他们的理论在实际中却毫无价值 | Yǒuxiē xuézhě zhǐ huì zhǐshàngtánbīng, tāmen de lǐlùn zài shíjì zhōng què háo wú jiàzhí
Some scholars can only theorize, and their theories are completely useless in practice
[Source]
大家动手干吧，不要纸上谈兵，浪费时间了 | Dàjiā dòngshǒu gàn ba, bùyào zhǐshàngtánbīng, làngfèi shíjiān le
Let's take action. There's no need for idle theorizing and wasting time anymore
[Source]
To engage in empty theorizing
None
You will see the character 赵 (zhào) a lot this story, so to summarize: 赵 is a the name of an ancient state/kingdom, and it's also the surname of two characters within the story (赵奢, or Zhào Shè, and 赵括, or Zhào Kuò)
《史记》(Shǐ Jì), or the famous work "Records of the Grand Historian" by 司马迁 (Sī Mǎ Qiān).
In the ancient state of Zhào (赵, 403 - 222) there lived a great general named 赵奢 (Zhào Shē). 赵奢 had a son called 赵括 (Zhào Kuò) who grew up extensively studying military texts, and by the time 赵括 reached adulthood, he had grown extremely arrogant and considered himself to have no equal in military strategy. Once, he even defeated his father in a debate of military matters.
The defeat in the debate and 赵括's overall behavior left 赵奢 extremely uneasy. His wife thought this odd, so she asked her husband about his reasoning, and he replied, "Warfare is a matter of life and death. Our son is very knowledgeable of the arts of war, but he doesn't have any real life experience. I fear that if he is ever actually sent to do battle, he personally will bring defeat to our army."
Some time later, the kingdom of Qín (秦) ordered its general 白起 (Bái Qǐ) to lead a massive army to attack the state of Zhào. The Zhào responded by sending the general 廉颇 (Lián Pō) to counter the attack, but the Zhào's relatively small army meant that they couldn't take the Qín head-on, and the general was forced to strategize. He knew that it was difficult for the Qín to transport provisions due to the area's geography, and that meant engaging in a long, prolonged war would be detrimental to them. He therefore ordered his men to prolong the war as much as possible by simply remaining inside their strongly-defended camp, ignoring all demands to fight, and ensuring that the Qín could not retreat.
For three years, this strategy worked perfectly, and the Qín army started to show signs of exhaustion. The Qín knew that they needed to get rid of 廉颇 if tides were to turn in their favor, so they sent troops out to spread a rumor that 白起 didn't fear the mighty general 廉颇 in the slightest, but that he was deathly afraid of 赵括. When the rumor made its way back to the king of Zhào, he immediately replaced 廉颇 with 赵括.
赵括's mother ferociously opposed the switch, and she told the king that her now-deceased husband had actually stated that his son must not set foot on a battlefield at all costs. However, her words fell on deaf ears, and 赵括 soon arrived at the front line.
The moment 廉颇 left, 赵括 proceeded to reverse his containment strategy by ordering a direct attack on the Qín. 白起 soon found out, and he had his men to pretend to lose, then double back to split the enemy army into two and cut off their supplies. 赵括 immediately fell into the trap, and about forty days later, the Zhào army was on the verge of starvation and its morale had all but collapsed. At this point, 赵括 tried to gain the upper hand by personally leading some elite troops onto the battlefield, but he was struck and killed by an arrow, and the entire Zhào army was forced to surrender. Unfortunately, the Qín were not merciful, and they responded to the surrender by having every one of the several hundred thousand warriors executed.