While the Polish–Lithuanian army was concentrated in
Podolia, in the south of the Commonwealth, a Swedish army landed near
Pärnu (today part of
Estonia) on 19 August 1621. It was carried by a fleet of 148 vessels (25 warships, 3 pinnacles, 7 galleys, 7 smaller warships and 106 transport boats). Gustav Adolf had 14,700 infantry, 3,150 cavalry and 375 cannons. The Swedes immediately marched towards
Riga, and started a siege on 29 August. The capital of Livonia was defended by a garrison of 300 soldiers, plus 3,700 armed residents. Lithuanian Field
Hetman Krzysztof Radziwiłł had only 1,500 soldiers at his disposal, as most units were in Podolia and
Red Ruthenia. Riga capitulated on 25 September, after three assaults. On 2 October the Swedes captured the fortress of
Dunamunde. The invaders, marching through marshes and forests to avoid Lithuanian cavalry, entered the
Duchy of Courland and Semigallia, capturing its capital,
Mitau without resistance. The Swedes then tried to capture the fortress of
Koknese, but failed to do so, after a skirmish with Lithuanian cavalry of
Aleksander Gosiewski. In early January 1622, the Swedes captured
Valmiera, together with several smaller Livonian castles. Meanwhile, the forces under Hetman Radziwiłł grew to 3,000, which enabled the Lithuanians to respond better to Swedish attacks. On 7 January Radziwiłł recaptured Mitau, but because of a lack of artillery, he failed to seize its castle, whose garrison did not capitulate until 6 July. A few weeks later, in late July 1622, main Swedish forces reached Mittau, and a prolonged battle ensued. Since the battle turned into a stalemate, on 10 August 1622, both sides signed a truce, which in 1623 was prolonged until March 1625. During the negotiations, envoys of Gustav Adolf suggested a Swedish–Lithuanian union. ==Truce==