Many Americans felt that Trump's ban was similar to an event in the 1940s. During the second world war, the Japanese attacked Pearl Harbour, an American naval base in Hawaii. Following this, thousands of American citizens of Japanese origin were rounded up and jailed in internment camps, suspected of being traitors. This became a matter of national shame later.
It is the memory of such historic wrongs, and the acknowledgment that their governments are not perfect and must always be challenged when they violate individual rights, that keeps America great. Perhaps Trump, who says he wants to 'make America great again' does not really understand the source of its greatness.