Spain has a 'right' to shoot down US aircraft  invading their airspace without consent

And now Spain actually said that we can't use their bases. And that's all right. We could use their bases if we want. We could just fly in and use it. Nobody's going to tell us not to use it.

According to international law, how could countries react if the US attempts to seize their military bases?

Well, they would have a right to shoot down American aircraft that were invading their airspace American aircraft that were invading their airspace without consent.