a small story which was told by Sringeri Acharya Sri Sri Sri Chandrasekhara Bharati Mahaswamulavaru. He was talking about Dasavataras and says [paraphrasing. so mistakes are mine. idea is His]

During Satya Yugam, Devas and Asuras were physically in different locations.. that is Devas were in Deva Lokam which was distinct from where Asuras lived in the the Asura Loka. A conflict would happen when any Asura wanted more than just his loka and went to occupy Deva Loka.

And then the Paramatman would wage a war almost always single handed.

Now coming to Treta Yuga, the Devas [Sri Rama] and Asuras were both in one loka. Some distant apart, but still one loka only.

Conflict would happen when, as in case of Satya Yugam, when the Asuras tried to disrupt the devas. In this case, Sri Rama went TO the asura loka be it Janasthana or Lanka and fought with them, along with help from other divine beings i.e vanaras.

This changed even further in Dwapara Yugam when the devas and asuras were born in the same family Sri Krishna & Balarama vs their mama Kamsa or Pandavas vs Kauravas. And the conflict arose as soon as they were born.