I believe North Korea will definitely invade South Korea if US forces withdraw from Korea.

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Currently, we rely on the United States for most of our North Korea surveillance systems and reconnaissance assets.
Even the concept of using the Kill Chain to launch preemptive strikes upon detecting signs of North Korean provocation is impossible for South Korea to detect on its own.
The withdrawal of US forces from Korea would essentially mean the end of the ROK-US alliance and a declaration that the United States is washing its hands of South Korea.
Why do wars happen? Most of them start due to a single person's miscalculation.
Most people thought Russia would finish Ukraine in a week and occupy it, and Putin likely believed that as well.
Just as the US turned Venezuela into a pro-US puppet regime in a matter of hours, people thought Iran wouldn't be able to hold out either.
If Kim Jong-un believes he can occupy South Korea, or at least believes he can gain something through war,
then war will begin. Current South Korea is even instilling that kind of belief in Kim Jong-un.
It doesn't matter at all whether South Korea's military power is stronger than North Korea's or whatever.
"South Korea, abandoned by the US? This is doable." If Kim Jong-un thinks this way, whether it is a miscalculation or not, war begins.