Sie sollen ihn nicht haben, den freien deutschen Rhein!
This map assumes a slightly more successful Napoleonic Empire that is able to retain Wallonia and parts of Switzerland. Napoleon survives politically and manages to settle peace with the British and the Russians, leaving France as the dominant power on the continent. The Germans are worried and Nationalism begins sooner to rise due to the stronger French position. The German Revolution of 1843 (1848 OTL) saw the unification of German lands and, most notably, the German-speaking part of Habsburg and Prussia. One of the most heard slogans was "Sie sollen ihn nicht haben, den freien deutschen Rhein!" (They shall not have it, the free German Rhine!) Vienna was under threat from Croatian, Hungarian and Polish nationalists and sought to secure its position by shifting her focus to Germany. In Frankfurt, the National Assembly chose the Austrian Emperor as the German Kaiser.
This unification triggered an agressive response from France which was deeply worried that its more-or-less hegemonial position on the continent was endangered. Following the German Wars (1844-1849), France acknowledged the German crown but gained the Rhineland (the United Provinces gained another part) from the exhausted and poorly organised Germans. This key region became a bone of contention between France, which continously sought to expand its borders to the Rhine, and Germany, which sought to prevent this and keep (or regain) the Rhineland.
Another major war between these two powerful countries, whose oszillation of relations determined the current affairs in Europe, broke out after the heirless death of Napoleon III in 1885 when the French revolted and proclaimed a liberal republic. Although the clash was mainly fought in the emerging colonies, and Germany was not able to regain the Rhineland, the war threw France into a deep political chaos which lasted two years and saw the radical liberals come into power. The II French Revolution thus resulted in a more stable, and liberal democracy.
Inevitably, at some point the two empires whose relations significantly warmed in the last decade of the 19 century, had to clash again. Most of the colonial wars, however, were contained and did not spread out to Europe. An exception was the Great War of 1916-1921 which saw almost all major powers involved and cost more than 6 million lives. The Germans could capture Paris in the end and their demand to cede the Rhineland to them was granted.
During the war, however, the constitutional system which has been set up 80 years ago in Frankfurt and made the German Empire a democracy, collapsed. The military leadership seized control of almost all important institutions while France, shaken at its foundations by the loss of the Rhineland and important colonies, remained liberal and repositioned itself on the global stage. Whereas the very strong and autoritarian German Empire became soon isolated in Europe, France allied with Poland and Hungary. German attempts to influence the United Provinces (Netherlands + Flandres) remained mostly unsuccessful.
The Strassbourg War broke out when a group of German pan-nationalists entered the border town and created a massacre for which the German government refused to apologise. The short conflict saw Germany attacked by both France and Poland, and after just a few months Frankfurt had to give up the Rhineland again in 1950. This time, military defeat led to an government overthrown in Germany: Workers and middle-class citizens were disillusioned with the military leadership which could not prevent defeat and, following popular demand, the military caste announced free elections which the ultra- and pannationalists overwhelmingly won. They did not openly campaign for war with France but the gain of the Rhineland, and perhaps more, was on their list. The relations with the Dutch, who controlled the part of the left Rhine northern of Cologne, however, relaxed and even became friendly.
In the 1960s, despite the Strassbourg conflict, both countries supported actively and passively militia groups which crossed the border and did valuable sabotage, espionage or attacks. Although relations improved a litte following the election of the Social Democrats in both Germany and France in 1962 and 1963 respectively, a deep economic crisis and the reemergence of pannationalism in the late 60s led to the number of border incidents hugely rising. To almost everyone it was obvious that the two countries were at the preface of war. Again.
