Not entirely true. Yes flags have always been used when tribes etc go to war.

But historically those same tribes often considered the result of winning a war. To include absorbing the population of the other. Tribe. Or the outsiders.

Many different attitudes towards those outside members have existed with different nations and tribes over time.

Vikings for example were nothing like the image we see in movies etc. those images were created by a single British priest as open properganda,

He is documented as having declared them as stealing UK woman by being over clean and inclusive of their wants and desires. (Paraphrased)

Rome was I little more questionable, but very much encouraged outsiders to join their culture. Not just as slaves. That generally only applied to opposition fighters. And those who refused to see Rome as the authority. But their Class system very much placed Rome born above others. But much like modern western nations. Immigration into Rome for employment as free citizens was very encouraged for those with any usable skills.