Complex sentence: The Paraguayan Fortín Nanawa was chosen as the main target of the Bolivian offensive since the capture of it and then the Paraguayan command centre at Isla Poí would allow Bolivia to reach the Paraguay River, putting the Paraguayan city of Concepción in danger.
Split into simpler sentences: The Paraguayan Fortín Nanawa was chosen as the main target of the Bolivian offensive to be followed by the command centre at Isla Poí. Their capture would allow Bolivia to reach the Paraguay River, putting the Paraguayan city of Concepción in danger.