Northern Europeans and E.Asians (and their descendants) have created the most successful cultures the world has ever known. It's not an accident as these populations have median IQs around 100 to 108. The remainder of the world has median IQs mostly in the the range of 75 to 89. See this map of estimated country IQs. The difference doesn't sound like much, but a median IQ of 85 would mean 50 times fewer brilliant persons (and 50 times more mentally deficient) in a given population than a country with a median IQ of 100.
It's a tale of two worlds. It's also a tale of the clash of civilizations.
The starkest example of this clash of civilizations is seen by comparing Israel with the Palestinians (and other Arabs in the region). Israel, full of higher IQ northern Europeans, is highly advanced and developed, prosperous, tolerant, democratic and highly civilized. The "Palestinians", like most other Arab/Muslim cultures, have devolved. Palestinians now find themselves locked behind walls due to their violence, are ruled by corrupt tyrants and completely dependent on international welfare to survive. That "welfare"creates a culture of victim-hood and dependence which furthers their cultural collapse, not unlike a substantial portion of US blacks. Palestinians could have decided decades ago, on any day, to make peace and work with/in Israel -- and enjoy the benefits of the only truly prosperous economy in that region. Instead, Palestinians have chosen intolerance and ruin for over 100 years now.
The entire Arab/Muslim culture in N. African and the Middle East has collapsed and a huge refugee crisis has emerged. Those "cultures" have devolved thanks to lower IQ and Islam. Islamic cultures are extremely backward; autocratic (like the religion itself) and weighed-down by self-imposed factors such as poor education, the suppression of women and violence, And there is little happiness in Islamic cultures (in the West at least). Religion is dangerous for people with weak minds! Their uncontrolled migration into Europe is a huge clash of civilization and will bring huge problems to Europe. Muslims in Europe are not integrating or adapting. They form "ghettos" of poor people on welfare and re-create the same failed cultures that caused them to flee to Europe in the first place! They have little to offer European civilizations in their devolved condition. Then, "welfare" causes even more cultural decline.
Africa is "the heart of darkness" when it comes to low IQs -- with many countries having median IQs in the 60s and 70s. Low black African intelligence extends to the US, Latin American and Caribbean black populations. Blacks have the lowest IQs in the world and it shows. Black persons in the US continue to be a huge burden to the US and to many countries in Latin America/Caribbean. Haiti is easily considered a failed state, for example.
Latinos, who have somewhat higher median IQs, due to historical intermixing with Europeans (median IQ of ~89 and a higher standard deviation according to "The Bell Curve" from 1995), are somewhat better off. But Latino economies are still mostly backward and poor and have high levels of crime. But Latinos, with intact families and family values, are more compatible with mainstream US culture. Even so, large uncontrolled immigration of Latinos into America will lower the average IQ of the US and reduce the performance of our economy on average; with lower income and more failed governance -- like what we see in Central and South America. Latinos fall for the false siren call of populist politicians who bring ruin to their countries. This is the mechanism for a large-scale decline of the US.
Western countries should acknowledge that immigration of large numbers of "3rd world" immigrants causes needless clashes of civilization and burdens to their citizens. Until recently, the entire history of the US includes nearly all immigration from high IQ areas of Northern Europe and it's been a source of great success. Some countries realize this. Japan allows virtually no immigration whatsoever and maintains a peaceful, homogeneous society. Sweden, on the other hand, has allowed large immigration of African and Middle Easterners in the past 2 decades and has become the "rape capital of the West" with higher crime and simmering unrest and resentment of ordinary Swedes. It's a powder keg now thanks to Leftist and Liberal politicians who are willfully blind and ignorant.
If Americans want to preserve our successful culture then, we should mostly limit our immigration for higher IQ persons from Northern Europe and East Asia. Also, America could interview and offer visas to interested PhD and Masters graduates, especially in STEM fields, to stay in the US regardless of country or region. Further large-scale Latino and 3rd World immigration should be limited if we want to limit a further "clash of civilization" in America. This is a prescription for preserving "American" culture.