I can say that Americans were not the only ones using religion as a justification. Enslavement at its heart is a class problem. The overclass creates some justification to exploit the underclass's labor. The most general expression is tribalism. "Our tribe good, other tribe wicked." And as tribal social organization evolved into cities, city-states, nations, multinational religions, etc, the scale of in group/out group evolved with them.
Ancient slavery absolutely used religion as a justification, because religious identity was the narrative tie for the in group. The old testament has passages of mosaic law that treated hebrew slaves like indentured servants, but allowed for permanent slaves from the canaanites. “Your male and female slaves are to come from the nations around you; from them you may buy slaves. You may also buy some of the temporary residents living among you and members of their clans born in your country, and they will become your property. You can bequeath them to your children as inherited property and can make them slaves for life, but you must not rule over your fellow Israelites ruthlessly” (
Leviticus 25:44–46).
European colonialism predates America, but had slavery as an economic and religious goal from the outset. Columbus claimed the Taino's were uncivilized and passive, and used horrific violence to oppress them. Every European colonial power participated in the African slave trade and called it "the white man's burden.". Besides African Americans, Americans also enslaved (and still do) indigenous people in their territories; Hawaii, Latin/South America, the Philippines, etc. In each instance, they justified their economic goals as "spreading democracy" or "spreading Christianity" or "civilizing the savages."
As to your second point about "whining about the lost cause," I think there are examples if we understand slavery as a form of economic oppression. Imperial Japan used slave labor in China/Korea/Okinawa/Hokkaido/Sakhalin before ww2, and there is a subset of Japanese nationalism that wants to bring back the glory of imperial japan. We can look at all the big capitalist global north countries and see that when capitalism is threatened, the ruling class becomes more fascist to oppress workers. See Germany/Italy/spain in the '30s, Great Britain in the 80's, Greece in the 2000's, Hungary now, the USA now. Sometimes this takes place not on home soil but in their tribute states; see every socialist country the USA toppled to install a pro-capital fascist over the last century; Nicaragua, El Salvador, Chile, Argentina, Palestine, Iran, Korea, Vietnam, Bangladesh, etc.
