Johan Bjornsson Printz was appointed third Governor of New Sweden in 1643, where he ruled until 1653. He arrived in February of that year at Fort Cristina with two ships: the Fama and the Svanen. He then ordered the construction of  Fort Nya Elfsborg on the East bank of the Delaware River near present day Salem, NJ, and Fort New Gothenburg on Timicum Island. With these establishments, New Sweden secured a monopoly of trade with the Native Americans, on both sides of the river, as far north is modern day Trenton.  These actions displaced the Dutch in the area, and caused increasing friction with those colonist as noted elsewhere in this timeline.

Fort Nya Elfsborg was named after the Alvsborg Fortress off Gottenburg, Sweden. The fortress was a Swedish style tree-cornered earthen redoubt. It was armed with 8, 12 pound iron and brass canons, which were mounted on earthen and wooden palisades. During this period, the river was mostly swamp, so the soldiers garrisoned there were besieged by mosquitos. This unfortunate fact caused the fort to be nicknamed Fort Mosquito. Due to this, the fort was eventually abandoned, and was burned down after the Dutch built Fort Casimir across the river in 1651.