what's the "heart land of China" in your last sentence above?
I've always thought there's Korea south to Yalu River, so did you perhaps mean some part of Korea historically claimed by China??
No it mean that any foreign army get too close to Chinese border it is interpreted as threat to China heartland of Beijing and Tianjin and China has the right to intervene and will
Here is a good reference on Korean war and dispel some myth about Korean war
19. What was the PVA and UN numerical strength at various stages of the Korean war?
Both sides had about the same numerical strength.
The first batch of PVA consisted of the 13th and 9th Army Group plus 3 artillery divisions, about 270,000 men.
At the peak time, total PVA and NKPA strength reached 1.2 million during the Korean war.
In June 1951, right after the 5th campaign, UN total strength was 695,110, with 520,850 ground force (US 253100, ROK 229600, other 38150), 80340 navy (US 66700, ROK 6000, other 7640) and 94520 air force (US 90000, ROK 4000, other 520). At the same time, PVA strength was about 240,000, much lower than total UN strength.
At the end of war, from PVA statistics, UN total strength was 1,111,340 (1.11134 million), ground force 904,550 (US 373,500, ROK 491,000, other 40050), navy 101,180 (US 73000, ROK 20000, other 8180), air force 105,610 (US 95000, ROK 10000, other 610). From western source (Korean Almanac), UN ground forces strength was slightly higher with a total of 932,539 (US 302483, ROK 590911, other 39145). PVA counted 70000 more US troops, this was probably because many ROKs (KATUSAs) were serving in US forces and counted as US strength by PVA.
In the war, PVA rotated about 2 million troops, US used 86% of its infantry and 14% of its Marine Corps, rotated over 1.319 million troops .
20. What is "human wave" tactics?
During battles, western forces were usually incapable of correctly estimating the strength of PVA forces, often times, they greatly exaggerated the number of attacking PVAs, such as taking a PVA regiment as a PVA division. PVA mostly attacked at night, blowing bugles and wistles, shouting thrills, even play "sweet music" to cause psychological stress, the PVA tactics made western forces feel that the enemy was everywhere from every direction. Moreover, PVAs were masters of infiltration, they often sneaked in and attacked directly on command posts, generating shock and chaos. Western combat history always refered PVA attacks as "swarm of Chinese", "human waves", "Chinese hordes", as if PVA simply threw its men into the fire and let itself slaughtered, such a description indicated a great misunderstanding of the PVA tactics.
As some military analysts pointed out, PLA rarely use dense formation in their attacks, it seeks to inflict maximum damage with mnimum casualty. At various stages of the Korean war, PVA nevered had a commanding numerical superioty against UN forces, in fact, during the 4th campaign, it was greatly outnumbered by UN (it was always outgunned), yet it could still outmaneuver UN forces and even managed to counter attack at X Corps. PVA could achieve all these with inferior firepower because it had smarter tactics and strategy.