In a free society such as ours, the burden of proof for imposing restrictions lies with the person or agency imposing the restrictions. Trump has not presented any credible evidence that the banned countries impose a risk to national security that justifies banning all citizens from those countries. The President does not and should not have the power to randomly ban travel from any country that he chooses...this is not a dictatorship. Yet, anyway. If you need more information, look it up. Reading the legal arguments and judgments for the previous ban might be a good place to start. And if you need more information about previous US experience with prejudicial travel and immigration bans that affected many local families, look up Yellow Peril.