On this edition of Africa 54: Ethiopian Prime Minister Abiy Ahmed says his federal forces withdrew from Tigray's capital Mekelle because it was no longer the "center of gravity."; the U.S. ambassador to the United Nations is calling on Russia to stop violence by "mercenaries working as an arm of Russia's Ministry of Defense" in Central African Republic; & Tanzanian President Samia Suluhu Hassan marked her first 100 days in office this week with the announcement of new measures in combating the COVID-19 pandemic.