This book (available from that link as a .pdf online for free) is a 1794* essay by Immanuel Kant on the possibility of ending war between sovereign nations. He basically argues that we need to seek to establish an international federation of co-dependent nations under a singular representative state. Pretty modern ideas for the 18th-century, but then, this is Kant we're talking about. His arguments are largely pragmatic and don't veer too much into philosophy** and should be generally digestible by a majority of readers. As stated repeatedly throughout the text, this is NOT a manifesto - I don't think Kant believed that any single state of policy would be able to even kickstart the move towards a perfectly peaceable world - but by holding out these plausibilities as ideals, he makes a very convincing case that establishing such a world is not beyond possibility even within a cynical grasp of reality, and so the main thrust of this test stands on its own two feet. Recommended reading for anyone whom this theme strikes curiosity into, but if you somehow happen to be a person of international political influence who reads this blog, I specifically implore you to read this and think of how Kantian your rationality as regards your work is.
* And the translation, by one M. Campbell Smith, was published in 1903 - so even the Very Lengthy (as in, longer than the translated text it was the introduction to) Introduction recounting the history of ideas around the core topic of this essay came too early to be able to speak of anything regarding such institutions as NATO, or the EU, or UN even, which might have quite substantively reshaped the commentary on the ideas herein.
** Except for the pair of appendices, where he first considers the disagreements between proper moral ethics and political reality, and then secondly looks at the singular overlap point between proper moral ethics and political reality - that being the idea of a public right.
