The war can be divided into four stages. In the first stage, certain commonwealth szlachta (nobility), encouraged by some Russian boyars (Russian aristocracy), but without the official consent of the Polish king Sigismund III Vasa, attempted to exploit Russia's weakness and intervene in its civil war by supporting the impostors for the tsardom, False Dmitriy I and later False Dmitriy II, against the crowned tsars, Boris Godunov and Vasili Shuiski. The first wave of the Polish intervention began in 1605 and ended in 1606 with the death of False Dmitri I. The second wave started in 1607 and lasted until 1609, when Tsar Vasili made a military alliance with Sweden. In response to this alliance, the Polish king Sigismund III decided to intervene officially and to declare war upon Russia, aiming to weaken Sweden's ally and to gain territorial concessions.