Internment of certain civilian populations (especially ethnic minorities) seems to have been a common occurrence during the 20th century. Examples include Boers in South Africa during the Second Boer War, Armenians in the Ottoman Empire during WWI, and people of Japanese descent in the US and Canada during WWII. I've heard that similar things happened in the Philippines under American occupation and in Cuba under Spanish rule.
How far back does this go? Why doesn't it go back to ancient times (if indeed it doesn't)?
A quick note regarding Cuban internment, since I'm not really qualified to speak on the other points you raised.
During the second Cuban war of independence (1895-1898) the Spanish government sent General Valeriano Weyler to put down Cuban insurrectionists. As it was essentially a guerrilla war, he employed a policy called the Reconcentración (Reconcentration). This policy consisted of insisting that all loyal Spanish subjects move to specialized camps in fortified urban areas, pre-approved by the government. Anyone caught in the countryside after a certain date would be considered either an insurrectionist.
While the policy lasted slightly under a year, the human cost was staggering, with some estimates totaling in the hundreds of thousands of civilian deaths due to lack of food, poor housing conditions, urban density in a territory notorious for contagious tropical diseases, etc.
Please note that while barbaric, Weyler's policy was not specifically targeting any ethnic group. It targeted people in the countryside as often being complicit in the insurrection through their support of the independence movement through commerce with or gifts to the rebels. In this way I don't think that you could call it a "wartime internment of ethnic minorities", unless by this you mean "non-whites", which while also true would misrepresent why they were being targeted. Cuba was not a majority non-white country of the likes of the Philippines or South Africa.