Complex sentence: Although Mississippi had a death penalty in 1963, it was unenforceable because it and other death penalty laws in force at the time had been declared unconstitutional by the United States Supreme Court in the case of "Furman v. Georgia".
Split into simpler sentences: Although Mississippi had a death penalty in 1963, it was unenforceable. At the time, the law establishing the sentence had been declared unconstitutional by the United States Supreme Court in the case of "Furman v. Georgia".