For the Spanish Armada to have conquered England in 1588 would not have been easy. King Philip's fleet would have needed several pieces of good fortune it did not get: a friendlier wind at Calais, perhaps, one that might have kept the English from launching their fireships against the Armada; and a falling-out between the Dutch and English that could have let the Duke of Parma put to sea from Dunkirk and join his army to the Duke of Medina Sidonia's fleet for the invasion of England. Getting Spanish soldiers across the Channel would have been the hard part. Had it been accomplished, the Spanish infantry, the best in the world at the time and commanded by a most able officer, very probably could have beaten Elizabeth's forces on land.
Harry Turtledove