The pre-eminent example for me would be the end of slavery in the United States, which was brought to an effective end by the American Civil War. Of course, force was not wholly responsible: decades of anti-slavery agitation played their part.
Other examples would be the birth(s) of democratic government in modern Europe, which involved multiple armed conflicts in Britain and on the European continent, from the English civil war of 1642-9 to the French revolution, to the rebellions of 1848.
Then there is the Russian Revolution; one can certainly criticize the Soviet regime which followed, but there's no question that it transformed Russian society very greatly.
The most appalling modern example is the Nazi holocaust, which shifted the demographic map of Europe (and Palestine, indirectly) as a result of the genocide against European Jews, not to mention all its very extensive and diverse political ramifications, which continue to affect today's headlines.
The old saw is that 'violence doesn't solve anything.' Maybe not, but it can sure transform existing problems into different ones.