Keeping out people from radicalized nations can only seem to decrease the odds of such attacks.

That is my guess.

But maybe Obama thought that letting in as many Muslims as possible would decrease the incidence of terrorism.  Assimilation, cultural exchange, listening, etc.   Winning hearts and minds.  And keeping Muslims out would make them more likely to radicalize.

Which is absolutely right?   I have no idea.

But we've given one remedy a very fair trial.  So has Europe.   And it's clear that new, very hard immigration standards are necessary.

These killings of American citizens are a very very serious matter to me.  Nothing could be more serious.