World war and immigration

As we go through the chapters of the book we see a pattern that during wars and crisis, the immigration would go down drastically. Even though United States was not involved in World War I or World War II well not till the end, why is immigration into United States being affected? Isn't it suppose to increase? Since people at their home country are at risk and they wanna go to a safer place, they could immigrate to America. Well this is just my concern and some one can think other wise. I wanna know if there were strict immigration laws during the time of war so people could not immigrate? I know after the war there were laws put in place and better regulation and quota systems were placed.