As soon as the frost had broken, the Swedish army commenced its advance. Skirmishes frequently took place, but Augustus had, as yet, no army with which he could meet them in the field, and he summoned a diet at Warsaw, in hopes of persuading the Poles to decide upon calling out the whole national force.

In this he failed altogether. The citizens, led by the foreign traders, were already openly opposed to him, and their attitude so encouraged his opponents in the diet, that many of these rose and openly denounced the government, and the conduct of the king, that had brought the country into its present difficulties.

As the Swedish army advanced, they were joined by the Duke of Holstein, and, in spite of the efforts of a considerable body of the enemy, under Prince Wisniowiski, progressed steadily, crossed the river Memel, and, when near Grodno, were met by an embassy sent by the diet, to endeavour to persuade Charles not to advance further.

An interview took place between the king, the Poles, and his ministers, the conversation on both sides being in Latin. But as the ambassadors had no definite plans to propose, and their leaders were wholly devoted to Augustus, the king refused to allow his advance to be arrested, and continued his march. When near Praga they crossed the plain where Charles Gustavus, King of Sweden, had defeated the Polish army in a great battle, that had lasted for three days. The city was occupied, and a contribution of 20,000 crowns imposed upon it, in addition to food for the army while it remained there. Plundering, however, was strictly forbidden, and, as the king issued a proclamation declaring that he was no enemy of the Polish Republic, but simply of their king, the inhabitants were, on the whole, well satisfied with the conduct of the invaders.

A halt was made here for some time, and a bridge was thrown across the Vistula, while the army rested after the long and fatiguing marches it had made. A fresh attempt was made to arrest the advance of the Swedes, and the Cardinal Primate, himself, met the king; but nothing came of the negotiations, and the army entered Warsaw. Here they were warmly received, and great entertainments were given to the king.

Towards the end of June, they again advanced to meet the force that Augustus had gathered, and on the 6th of July the Swedes arrived within a few miles of Clissow. The next day some reinforcements arrived, and the king decided to give battle on the following day, which was the anniversary of the victory on the Dwina, the previous year.

His army was twelve thousand strong, while that of Augustus was nearly double that strength, and was very strongly posted, his camp being surrounded by morasses, although situated on rising ground which commanded the whole of the country round it. The bogs in the front were found to be so impassable, that the Swedes were forced to make a circuit to the left, where the ground was firmer. This movement obliged the enemy also to change front, a movement that caused considerable confusion, as they themselves were forced to traverse boggy ground, to take up a new position facing that by which the Swedes would now advance.

The attack was commenced by the division commanded by the Duke of Holstein, but, scarcely had he set his troops in motion than he was mortally wounded, by a ball from a falconet. His troops, however, pushed forward vigorously. The Polish division opposed to them resisted the two first assaults bravely, but gave way at the third attack, and were driven from the ground, in such confusion that they took no further part in the engagement.

While this was going on, the Saxon cavalry had been repulsed by that of Charles, and, passing in their retreat under the fire of three infantry regiments, suffered so heavily that they left the field. The Swedish foot now advanced all along the line, and in the centre destroyed several battalions of Saxons.

But the Swedish right was attacked so vigorously by the Saxon left, under Field Marshal Steinau, that for a time the conflict was doubtful. The Swedish horse guards and other cavalry, however, charged with such determination that the Saxon horse on this flank were also defeated, and driven off the field, while the Swedish infantry, advancing without firing, drove several battalions of Saxon foot into a village, where, being surrounded, almost all were killed or taken prisoners.