You can really read the entire history of Europe from roughly 1800 to today as one long struggle to answer the "German question" (
https://en.wikipedia.org/wiki/German_Question): should there be a unified state for German people, instead of having them scattered across lots of tiny, independent principalities? And if the answer to that is "yes," where do you draw the lines of that state's borders?
It was a central question to the politics of the age, for the simple reason that it was always obvious that by gathering so much of the continent's population under a single flag a unified German state would immediately become one of Europe's most powerful. Until the rise of German nationalism (https://en.wikipedia.org/wiki/German_nationalism), the largest and most powerful state in Europe had been France, and Napoleon had tapped that power to make the French the masters of the continent. A unified German state would be large enough to be a counterweight to French power, or even eclipse it altogether. So the German question became the fulcrum on which power politics in Europe balanced.
Obviously the French, along with the rest of Germany's neighbors, had an interest in keeping the boundaries of a German state as limited as it could be. But there were divisions of opinion within the Germanic world about how big a German state should be as well. The maximalist position was the "big German solution" (Großdeutsche Lösung), which advocated pulling all German-speaking people, including those in Austria, into the new state. Advocates of the competing "little German solution" (Kleindeutsche Lösung) were more modest; they simply proposed unifying the northern German states.
This argument reflected the division of power in the Germanic world itself, which was split between Prussia in the north and Austria in the south. Austria then was much larger than it is today, part of the enormous kingdom of the House of Hapsburg (see https://en.wikipedia.org/wiki/Habsburg_Monarchy), so the Austrians tended to support the "big solution", since it was assumed that if Austria was part of the German coalition it would dominate its smaller partners. For the same reason, the Prussians favored the "little solution"; they wanted to dominate the united Germany, and the only way they could do so was by keeping the Austrians out.
All of this got worked out in a series of wars and revolutions between 1848 and 1871, during which Prussia beat first Austria and then France soundly on the battlefield. These victories put the Prussians in the driver's seat in terms of determining what a unified Germany would look like, and since the Prussians supported the "little solution," that was the shape the new German Empire took.
As they had hoped, once Germany took the form they wanted, the Prussians more or less ran the Empire. But then came World War I, and Germany's bloodletting and defeat in that war knocked the Prussians off their pedestal. That created a power vacuum, which was eventually filled by Adolf Hitler and the Nazi state. And unlike the Prussians, Hitler was very much a believer in the "big German solution" -- so much so that his "Greater Germany" encompassed more people than even the most strident 19th century German nationalists imagined.
One of the arguments for the "little solution" had always been that making Germany too big would unite the rest of Europe in an effort to cut it down to size, as had happened to the French during the Napoleonic era; and Hitler's wild expansion proved that was the case. And once the war was over and "Greater Germany" had been conclusively defeated, those nations (or rather the United States and Soviet Union, who had been drawn into the issue because Hitler's ambitions were so large that they had spilled over the boundaries of Europe altogether) moved conclusively to split Germany into two states so it could not amass enough power to threaten the peace again.
That was how things were until 1991, when the Soviet Union collapsed and with it its East German puppet government. West and East Germany then moved to reunite, and since nobody in the NATO coalition had any objections, the modern German state was born. This was a "little solution" Germany; the war and its aftermath had taken the shine off the idea of bringing all Germans everywhere together by force of arms. But when the European Union began to come together, this "little Germany" was still plenty big enough to dominate that confederation, especially with the UK staying out. So we reach the modern age, in which Germany is the unquestioned leader in European politics -- though only as long as the EU remains the form in which those politics are expressed, which isn't anything like a sure thing.