It's not simple. De Valera as President of the declared Irish Republic sent Michael Collins to London to negotiate with the British Government the conditions of the Treaty. On return to Dublin, De Valera caused a split in the ranks of the Irish deputies by voting against the terms of the treaty. This in turn lead to the Irish Civil War in which Michael Collins was killed. One of the saddest days in the long history of Ireland. The bitterness of the conflict still has echoes in Ireland today. A full explanation can be found in the books by Tim Pat Coogan on both Michael Collins and De Valera. Both are well worth the effort. M