Do you want to know why the worldwide terrorist attacks were happening in other parts of the world? Because the main target of radical Islamist terrorists are Muslim countries. We may all think that their only goal is to kill Americans, but in truth ISIS and all the bad hombres out there are focused on their homeland and the countries that surround them. They have no hope in hell of growing without gaining territory. This is why going there to fight them makes more sense than trying to block every Muslim for entering the US. The threat is real, it's just not here.
The time and energy spent against these theoretical "in-unison" terrorist attacks on America could be much better spent doing other things to save lives and help people in your own country. Things that take less money and time and energy. Helping veterans that are struggling (way more veterans kill themselves than Americans are killed by terrorists). Wouldn't that make more sense? There are large problems in the US that are costing thousands of lives every year, and there are tangible ways to help those people... but still, the number one concern of the US government right now is stopping an immigrant from Yemin from getting into America because he might be a terrorist.
Use your resources to fight the things that are real and have solutions. If terrorists want to terrorize you, they will one way or another. Regardless of the steps and measures you take. They have people living among you, and those people make up the recent terrorist attacks in America. Try stopping that.