In March of 2016, presidential candidate Donald Trump was asked: "Your slogan is 'Make America Great Again,' When was America actually great?" Trump responded that America was last great in the late forties and the fifties. Sorry Donald; I remember that period and it wasn't great.
Trump explained that after World War II: "We were not pushed around, we were respected by everybody... we were pretty much doing what we had to do." (http://www.cnn.com/2016/03/26/politics/donald-trump-when-america-was-great/index.html) Most Trump voters agree with this sentiment (https://www.theatlantic.com/politics/archive/2016/05/make-the-sixties-great-again/481167/), but their response is influenced by when they were born -- for example, Trump supporters born in the sixties think the eighties were great.
As to be expected, Trump's recollection of the fifties is way off. He recalls, "We were not pushed around, we were respected by everybody..." But this was the era of the Cold War with Russia (U.S.S.R.). Trump conveniently forgets the "Iron Curtain" and the threat of nuclear war. (Many of us, who lived through that period, remember "duck and cover" exercises where students prepared for a Russian nuclear attack.) The fifties era was dominated by anti-communist rhetoric. There was a "Red scare" led by anti-communist zealots such as Senator Joseph McCarthy.
Now Trump wants to normalize relations with Russia and replace the Cold War, in the public consciousness, with the threat of a global war with terrorists. Trump has combined this dangerous image with his vision of an invasion by undocumented immigrants; Trump's obsession with building a wall along the southern border stems in large part from his obsession with these immigrants. Thus Trump would replace the "Red scare" of the fifties with a new "brown scare."
Trump recalls the fifties as a period where, "we were pretty much doing what we had to do." He's ignoring the fact that a huge portion of the world -- the USSR and mainland China -- was "off limits" to Americans. Nonetheless, during the fifties US corporations dominated trade in "the free world." (We came out of World War II with a robust economy whereas the economies of most of our allies had been decimated by the war.) It's understandable that Trump, and his supporters, long for a simpler time when the US economy ran the world and and American companies dominated trade. Realistically, that time is long gone. We now live in a much different, global economy.
One way to interpret Trump's comments, "We were not pushed around, we were respected by everybody," is that he is referring to the United States. Another way to interpret this remark is that he is referring to white folks, white men in particular. Indeed, the fifties represents the zenith of what UC professor George Lakoff has described as the "strict father" morality: "In the strict father family, father knows best. He knows right from wrong and has the ultimate authority to make sure his children and his spouse do what he says... This reasoning shows up in conservative politics in which the poor are seen as lazy and undeserving, and the rich as deserving their wealth." In many regards, from a cultural standpoint, the fifties was the last decade where white men reigned supreme.
Obviously, Trump ignores the fact that the late forties and early fifties witnessed a resurgence of white supremacy -- which had been somewhat muted during the war years. After the end of World War II, "Jim Crow" laws were strictly enforced in most parts of the country and many people of color were forced to use segregated facilities. (The initial battle against segregation began in 1955 with the Montgomery bus boycott and culminated in the 1964 Civil Rights Act.) In general, the late forties and early fifties was a period of unfettered racism and sexism. (Until the Trump era, this was the last period where hate and bigotry were considered politically correct.)
So, when Trump says he want to "Make America Great Again," he's calling for a return to the mentality of the fifties. In terms of foreign policy, he wants America to be the top dog, to once again be the world's policeman. And in terms of domestic policy, he's calling for a return to the era of white supremacy, to the period where straight protestant white males ruled American cultural life. Trump is calling for an end to "political correctness" and, indeed, for an end to everyday decency.
Trump doesn't actually want to "Make America Great Again," his intention is to "Make America Hate Again."