Starting in the 1670s, Louis XIV established the so-called Chambers of Reunion, courts in which judges would determine whether certain Habsburg territories belonged rightfully to France. The king was relying on the somewhat vague wording in the Treaty of Westphalia, while also dredging up older French claims, some dating back to medieval times. Through this, he concluded that the strategically important imperial city of Strassburg should have gone to France in 1648. In September 1681, French troops occupied the city, which was at once strongly fortified. As the imperial armies were then busy fighting the Ottoman Empire, they could not do anything about this for a number of years. The basic aim of Louis' foreign policy was to give France more easily defensible borders, and to eliminate weak spots (Strassburg had often been used by the Habsburgs as a gateway into France).