When humans and aliens arrive

What if humans and aliens simultaneously arrive at some unsettled planet?  Ah... now things get interesting.

This scenario isn't one-sided like the previous ones.  The two species are both capable of interstellar flight, so they are presumably equally advanced.  It would be the dawn of a new cold war.

Of course, there's a small chance that interstellar travel is just a fluke development for one of the species.  In that case, there could very well be a significant gap in technology.  Harry Turtledove wrote a story with that premise.

But I think the Cold War analogy is the likeliest.  The sides are roughly equal, and there would be a lot of posturing from both of them.  Lines would be drawn, and Berlin Walls erected.  Spying and paranoia would reign supreme.  Most importantly, each side would build up its military as much as it could.

Eventually, one side would begin to pull away from the other.  That's when the real action begins.  If there's a form of Mutual Assured Destruction, making war unwinnable, then the laggard would simply implode, both politically and economically.  If there's not a form of MAD, and the laggard thinks there's a chance, however small, that it could win a shooting war, well... goodbye, cold war, and hello, Interstellar War I.

The good thing about this scenario is that it provides the best ground for fiction.  The humans can win, but we can also lose, and there's the dramatic tension.  As long as the war goes on, we can splatter alien guts with our plasma rifles.  We can nuke their cities from orbit.  We can cleanse whole planets of their filthy hides.

And that's just good clean fun.