Great Qin would only be sending 100 million soldiers to the back, and most of the City Lords would be at the front to draw attention. As such, simultaneously invading four countries would be quite difficult, so Zhao Fu planned to invade one country first instead of splitting his forces.
After destroying a country, they would immediately turn to another country. They would first destroy Myanmar and Thailand, as they were stronger than the other countries.
Because Laos and Cambodia were weaker, after Great Qin destroyed Myanmar and Thailand, dealing with them would be much easier.
Moreover, Great Qin had a massive chess piece buried within Laos, which was the Vietnamese faction from before. More than 70% of the factions there had secretly decided to join Great Qin, and Zhao Fu could order them to attack Laos at any time, helping Great Qin easily take down Laos.
As such, with only Cambodia remaining, it would not have any hope and could only be destroyed.