I will try to flesh this out in a little more detail with historical examples, but I need to get some sleep first. So I shall say this:
The worry about the Japanese spies didn't confirm itself because the Americans were too paranoid about being attacked on their homeland after what had happened at Pearl Harbour, so they provided for themselves a buffer in order to ease their resentment of the Japanese and to ensure that any existential threat from within was almost non-existent. Tensions were extremely high, as the entire country was pulled away by the tide of the biggest war in human history; the entire industry was mobilised on a scale that had never been seen before in its history. A B-25 Mitchell light-bomber rolled off the production line every 60 minutes. The issue with the tensions came about because the ideas of American exceptionalism during that time were higher than they had ever been: the citizens viewed their enemies as inferior in the sense of cultural habits and customs, and as the war in the pacific began to grudge along like a meat-grinder. Not only was the end of the war against the Japanese feeling like an extremely slow and grinding possibility, the exceptionalism attitude towards America's neighbours, both close and further away, was tested more than it had ever been in the history of the Republic. American superiority in technology, industrial capacity and patriotism became increasingly overshadowed to the utter bewilderment and horror of the United States Marine Corps as they fought their way across Guadalcanal in 1942. But the real turning point in the pacific came during the aftermath of the Battle of Tarawa in 1943.
The American invasion force to the Gilbert Islands was the first major offensive as part of the "Island Hopping" campaign after the decisive victory during the American counter-attack at Midway; destroying the Japanese carrier-based capability to a significant extent. The fleet headed for the invasion was the largest yet assembled for a single operation in the Pacific, consisting of 130+ ships and 17 aircraft carriers. On board the transport ships was the 2nd Marine Division and a part of the Army's 27th Infantry Division, for a total of about 35,000 troops. For several hours, the northern tip island - Tarawa - consisting of only 600 yards of palm tree-laden airstrip was heavily fortified by 2,600 Imperial Japanese Marines. As the Marines approached the beach, they began to realise that the Japanese were fighting on a scale of ferocity that had produced, during this battle, some of the fiercest and most intense fighting the Marines would ever had to have faced. 5,000 Marines managed to make it ashore, and after 72 straight hours of intense fighting, only a handful of Japanese were left alive and taken prisoner; the rest had died for their emperor. For only 600 yards of sand, one thousand Marines were killed, and 500 further casualties. The aftermath was shocking for the Americans, and it was the first time in its history to have publicly screened combat footage of the fighting, as well as displaying photographs of dead Marines lying on the beaches.
The combined reaction to such a battle, and the many battles that were fought for much longer, in harsher conditions, and with far more brutal casualties, produced a shock-wave of resentment for the Japanese that lasted for years even after the war finally ended. Some veterans admitted to hating the Japanese for decades post-war for the brutal lengths they went to ensure that their honour remained intact for the Emperor's glory. During Okinawa, some Japanese troops strapped bomb belts to the coerced civilians, and forced them down the rocky hills toward the entrenched Marines sitting in the mud hoping not to get their throats slit. Heinous atrocities occurred that would make your blood run cold and your heart skip a beat. It must be maintained that the Japanese threat during the Second World War was wholly existential. The Japanese Americans considered themselves simply American at heart and in their minds. They had absolutely no hidden agenda toward their fellow citizens, and the American veterans often discussed the rising regret they had felt in allowing the internment camps to continue through the war, despite them even being allowed to fight for a country that had treated them in a disgusting manner back home.
Having said all of that...It is unwise to compare the mistrust and mistreatment of the Japanese Americans during the Second World War between that and the fears that many Europeans now currently face as a result of the recent influx of nearly a million migrants and refugees; most of whom had not fled from war-torn Syria, but had cross through EU countries from North Africa in order to better their economic interests. Many of these migrants are currently showing absolutely no signs of wishing to integrate within their respective EU countries. Many of them have been filmed setting fire to streets, attacking journalists, attempting to raise Jihadi revolts, raping women en-masse in cities such as Colonge, and refusing to be given food and water upon entering the country. The media preferred to keep these events under tight concealment so as not to 'stir the pot'. It is slowly beginning to dawn on the European population that Islamic ideology and the many sub-sections of its cultural values within the North African and Middle-Eastern states is being exported wholesale to the Western continent on a scale and at such a rate, that a battle between civilisations and the questions raised from it is almost too inevitable to call. There is no immediate existential threat by a unified state governed by autocratic measures such as that of the Imperial Japanese Empire during the 1940s. But the ferocity and determination to protect their identity whilst crushing another, to project their own prejudices of women, of gays, of Jews, and of non-Muslims upon the citizens of the countries they find themselves thrust into has led to some horrific events unfolding in the few months. Western civilization needs to be preserved, and the EU-led project to commit to open borders policy at the expense of the host cultures has created yet more resentment of the established political class; which has witnessed a sudden halt with Britain voting to leave the European Union two days ago. This is a battle between conflicting civilizations; between a unified continent based on shared liberal values of tolerance and individual liberty, and of a grouping of countries which cannot control itself due to sectarian turmoil and political sabotage plagued by civil war. If such ideologies are being exported to countries in-which most of the population is not even considering itself Muslim, deep tensions are proving to be an inevitable consequence of no border controls. Combine this with the fact that Islam has been locked into a civil within its own ranks; between Sunni and Shia, and different regions and nations of Islam battling to be at the centre of the Prophet's holy order. Nothing good at all can come from this cultural and religious violence exporting itself to the wider regions that have been relatively peaceful in recent years.
I'll try to answer your question in further detail after I get some sleep
