According to the archives, the State of Zhao at the time was the only other Kingdom in China that had the sufficient military power to oppose Qin, particularly in Northern China.

Zhao's military force was so imposing, that Qin King sent 1 of his grandsons to be a hostage in the state of Zhao (before the Battle of Changping).

Zhao's military force was strategically important to the defense of Northern China against further Qin expansion, because Zhao by its location may quickly dispatch aid to neighboring state of Han and Wei to stop Qin. This occurred number of times before.

At Battle of Changping, Zhao's defeat meant the loss of virtually an entire generation of Zhao's most trained soldiers and generals. Zhao could never recover from this defeat. While Qin was then able to expand unhindered across northern China.

After that, the ONLY 2 kingdoms large enough to oppose Qin was the eastern state of Qi and Southern state of Chu.

Qi was mired by internal corruption and acquiesced to Qin's expansions.

Chu did put up a fight, in 223 BC and 222 BC. Chu mustered some 500,000 (yes, Half Million) troops, a massive army to repel a Qin invasion of 200,000 troops, resulting in a bloody victory over Qin.

But Qin mustered a 2nd invasion force of 600,000 troops to march on Chu, and Chu was defeated and conquered.

In that 2nd invasion, though Chu forces were only slightly fewer than Qin forces, Chu (southern) soldiers are generally thought to be less trained and less well equipped foot soldiers, while Qin forces were more trained, more diversified in tactics, and had large number of strong cavalry units.