What could we unite and do against a solar flare? We get some advanced warning but not that much. The sun had a massive solar flare just in this past week that we got lucky was on the opposite side of our orbit whose shockwave still hit the Earth, and that can reach us in about 30 minutes according to the following article. That's not a lot of time to unite and take action.
"Even though the CME erupted from the opposite side of the Sun, its impacts were felt at Earth. As CMEs blast through space, they create a shockwave that can accelerate particles along the CME’s path to incredible speeds, much the way surfers are pushed along by an incoming ocean wave. Known as solar energetic particles, or SEPs, these speedy particles can make the 93-million-mile journey from the Sun to Earth in around 30 minutes."[1]
I'd like to hear opinions on other thing. would nations unite against alien invasion? a solar flare? killer robots?