lit. military tactics on paper (idiom)

fig. theoretical discussion that is worse than useless in practice

armchair strategist

idle theorizing

cf Zhao Kuo 趙括|赵括[Zhào Kuò] leading an army of 400,000 to total annihilation at battle of Changping 長平之戰|长平之战[Cháng píng zhī Zhàn] in 260 BC