Lots of people like to think about alien vessels, UFOs, in a positive light. But very few though about how would world powers would actually react to dangerous aliens, like something that could just drill into your advanced air defenses without leaving any trace all available sensors.
One possible outcome could be just to agree to keep their presence under plausible deniability, just till something lands somewhere infront thousands of witnesses (considering our simulated scenario, based in the current situation, "very public" actions are not the alien's playbook).
The other obvious outcome should be to precisely coordinate military presence in the planet, trying to cover any territory with some kind of military response - the ASAP one+highly guarded airspace close to key infrastructure/cities in the planet - in case of detections / ingress of UFOs.
Kind of obvious, you would need to agree with ALL the players, Russian Fed, China, India, and a dozen more. The nuclear players with missiles capable of targetting stuff in low orbits should be the premium resources,
Space related industry should be developed at accelerated pace, and attempts to reach close things in the solar system would be at some point almost a monthly thing.
There are a tons of ones, many which you won't find in any scifi book, because the scenario where you just watch UFOs (IF you can detect them), trying to do something on the planet without knowing WTF is going on for decades is not a compelling base for anything entertaining