Unbeknownst to most fans, many Sega games saw a release on Nintendo's system of choice, the Nintendo Entertainment System (or Family Computer in Japan). Sunsoft brought several games to Nintendo's console in Japan (and Asmik brought at least one), while Tengen (Atari's home console division) brought many Sega System 16 ports to North America. Some of Tengen's releases, like Alien Syndrome, were modified versions of the Sunsoft releases.
It is not known exactly what the positions of Sega and Nintendo were when it came to the North American market. Tengen brought its games to the NES without the Nintendo seal of approval, and often with chips that would bypass the piracy prevention tactics Nintendo had adopted. Nintendo were hence often engaged in legal battles with Tengen (especially when it came to the release of Tetris, in which both Nintendo and Tengen had released a version). Tengen's carts were not distributed in large numbers, and many of their Sega ports have become rare collector's items as a result.
On the other side of things, most of these games were released in about 1989/1990, at a time where NES sales were about to decline thanks to the Sega Mega Drive. Many these ports were also several years out of date, with superior Sega Master System versions appearing much earlier, and the quality of these titles were not always on par with Sega's works.