 Complete the following phrase:  The fort was constructed in 1863-64 during the Civil War as an earthwork battery on the south shore of the mouth of the Columbia River, and was known as the Fort at Point Adams. It was later Fort Stevens in 1865, in honor of the former territorial governor of Washington, Isaac Stevens, who had been killed in action. Fort Stevens was the primary military installation in what became the Three Fort Harbor Defense System at the mouth of the Columbia River. The other forts were the Post at Cape Disappointment, later Fort Cape Disappointment and later Fort Canby, built at the same time as Fort Stevens, and Fort Columbia, built between 1896 and 1904. Both are on the Washington side of the river. The fort was meant to defend the mouth of the Columbia from potential British attack during the 