For most of the junior league’s life span, the NBA and ABA had a testy relationship. The ABA did a lot of things the NBA did not like at all, like introducing the three point shot and having a flashier, fun focused game compared to the NBA’s stodgier, walk-it-up style. Plus the ABA _really_ irritated the NBA when it started competing harder for talent by drafting younger and younger players. The ABA undercut the NBA’s hardship system and later drafted high school players like Moses Malone.
There was, however, a relatively brief period where the NBA and ABA sort of got along - or at least the players did.
The respective players associations organized two NBA-ABA All Star games in 1971 and 1972. This was against the wishes of the NBA owners, who were frustrated by the costs of competing with the ABA. For its part, the ABA was running out of money and wanted a merger, something the NBA had also come to see as desirable.
The players organized two games but in 1974, the NBA owners ultimately won a binding arbitration ruling that put an end to it.
[In this game from 1972](https://www.youtube.com/watch?v=hjCd8anskXo), you see Julius Erving, Rick Barry and Artis Gilmore for the ABA, among others, and Wilt Chamberlain, Oscar Robertson and Kareem Abdul-Jabbar among the greats representing the NBA.
It was an interesting idea and too bad the NBA shot it down.
Incidentally, the NBA won 125-120.