OCEANSIDE — This fall, the last American troops will leave Afghanistan, ending a 20-year war that involved tens of thousands of troops who trained and deployed from Marine Corps Base Camp Pendleton. Since the 200-square-mile military base was established along the northern border of Oceanside in 1942, the city has had a strong and symbiotic relationship with the Marine Corps. But the Corps’ economic impact on the Oceanside economy has gradually declined. Over the past two decades, the city’s fast-expanding tourism industry has outpaced the military as the primary driver of business. Longtime Oceanside merchants and residents say the city’s downtown area now is nothing like it was in October 2001, when the first troops deployed to Afghanistan. In the years since then, downtown’s last strip club and the rowdiest of its bars have all been replaced by restaurants, high-end condominiums and hotels.