How did countries in ancient China communicate with each other when they were at war? At that time, there was no Mandarin, and I could understand it everywhere.

Do you still need language to fight? It's just to attack our frontier, but there must be "Mandarin" in ancient times. As for what kind of language, we can't understand it now. In fact, there are still a lot of ancient "Putonghua" in the southern dialect of China, but only a few words. You can find a lot of information about this on the Internet. The following passage comes from Baidu dialect encyclopedia (plagiarism).

Fuzhou dialect has many sources. Ancient Chinese and Middle Chinese obviously remain in Fuzhou dialect, which is mainly due to the large-scale immigration during the Han, Jin and Tang Dynasties, as well as the remnants of Guqi and Gujin languages (for example, Fuzhou dialect has the truncation words of modern Jin language), and these sources gradually merged to form the current Fuzhou dialect. In the past 300 years, great changes have taken place in vowels, phonetic changes and vocabulary of Fuzhou dialect, but this change is extremely slow.