commonwealth power began waning after a series of blows during the following decades. a major rebellion of ukrainian cossacks in the southeastern portion of the commonwealth (the khmelnytskyi uprising in modern-day ukraine) began in 1648. it resulted in a ukrainian request, under the terms of the treaty of pereyaslav, for protection by the russian tsar. russian annexation of part of ukraine gradually supplanted polish influence. the other blow to the commonwealth was a swedish invasion in 1655, known as the deluge, which was supported by troops of transylvanian duke george ii rákóczi and frederick william, elector of brandenburg. the tatars of the crimean khanate and the nogai horde conducted almost annual slave-raids in the eastern territories controlled by the commonwealth.