Marcus Garvey's Back-To-Africa argument encouraged those of African descent to be proud of their race and to return to Africa. Garvey even founded, in 1919, a steamship corporation called the Black Star Line, to provide transportation to Africa, for those who wanted to return.
In a speech in 1921, Garvey estimated that there were approximately 400 million people in the world of African descent, and argued that "the time (had) come to unite these 400 million people toward the one common purpose of bettering their condition." Garvey thought that if all of these 400 million Africans came together, in one place, then they would first of all be free, where in may cases they were not at the time free, and that, secondly, they would have the strength in numbers to affect real, lasting change, part of which would be economic self-sufficiency. Garvey also argued that unity between the races was never going to happen, and that, therefore, the choice to relocate and unite in Africa was the only real choice that African Americans had if they wanted to achieve freedom and self-sufficiency.
It's easy to imagine why, in the early part of the twentieth century, Garvey might have thought that unity between white and black people was an impossible ideal. This was a time in which the Ku Klux Klan were increasing in popularity, black people were being lynched, and segregation was institutionalized. The logic behind his other arguments, noted above, also seems to be relatively sound. Four hundred million people working together for one uniting cause would surely have been a powerful and impactful movement.
One important argument against Garvey's Back-To-Africa movement, however, is that for many millions of African-Americans, America was the country they considered to be their home. To accept Garvey's proposal, and return to Africa, the land of their descendants, would to some extent have been to tacitly approve of, and so lend some degree of false legitimacy to the argument put forward by white supremacists like the Ku Klux Klan, that America was for white Americans and that African Americans didn't belong. For millions of African Americans, America was their home, and not Africa. More than 350 thousand African Americans fought for the United States of America during World War 1. They had invested in their country, and, in my opinion, rightly felt that this was the home they should fight for, rather than moving their lives to start again somewhere else.