ISIS apparently warned the Brits that there were attacks coming and I don't exactly know what they could have done. It's not like you can have an officer on every corner or respond to every threat.
Here is my solution; expect an attack every day.
For the US enact the Presidents travel ban and don't import refugees from terror sponsoring countries. These people can not be vetted... period. Isolate subversives in the country and throw them out.
Leaders better wake up because people are getting sick of the PC political response. Political Correctness is getting people killed.