In the third sentence of the first paragraph, the author mentions that war "has, thought the centuries, been recognized by international law as a sovereign right" primarily to

Historically the concept Jus ad Bellum (lat. 'right for war') refers to the right of monarch or state to start a war against another political entity. Until the beginning of the 17th century, this right was absolute, meanwhile after the Hugo Grotius (1583 - 1645) published the book 'On the Law of War and Peace' in 1625, the right for war was limited with international law.

Explanation:

In particular, a set of criteria was introduced to verify if the war can be qualified as a just war (morally justified war). They were introduced to prevent states to start a war only with intention to gain more territories, human and other resources (among other assets). Opposing, for example, to the protective war. Meanwhile, the concept of just war often gets

stretched.