Louis' advisors believed that, as long as Strasbourg remained independent, it would endanger the King's newly annexed territories in Alsace, and, that to defend these large rural lands effectively, a garrison had to be placed in towns such as Strasbourg. Indeed, the bridge over the Rhine at Strasbourg had been used repeatedly by Imperial (Holy Roman Empire) forces, and three times during the Franco-Dutch War Strasbourg had served as a gateway for Imperial invasions into Alsace. In September 1681 Louis' forces, though lacking a clear casus belli, surrounded the city with overwhelming force. After some negotiation, Louis marched into the city unopposed on 30 September 1681 and proclaimed its annexation.