Grotius ...
Hugo Grotius (1583-1645), Dutch jurist and statesman, and author of On the Rights of War and Peace (1624). In his early work Rousseau wrote approvingly of Grotius' concept of natural law, but by the time he came to draft his "Political Institutions" that led to On Social Contract, Grotius like Hobbes, had become a prime object of Rousseau's attack.
Hobbes' portrayal in Leviathan of a "natural" war of "every man against every man and Grotius' discussions in The Rights of War and Peace of "private war" and "public war" provoked Rousseau's assertion, in his unpublished manuscript on "The State of War" and in Book I of On Social Contract that there is no such thing as "natural" or "private" war. Since human beings are "naturally peaceful and shy" they do not engage in war. War, which is based on the sustained and rational will to harm another, is something that only states, not individuals, engage in.
Another aspect of Hobbes' and Grotius' writings problematical for Rousseau was their argument that those conquered in a war could legitimately be enslaved by the conquerors. In Chapter 20 of Leviathan Hobbes asserts outright that despotic force is a legitimate means of establishing sovereignty.
Finally Rousseau disputed Grotius' assertion, based on Biblical examples, that individuals or nations had the right to submit voluntarily to slavery. Rousseau argues forcefully that since human beings are naturally free, they do not have the right to enslave themselves.
By Book I Chapter 4 of On Social Contract, Rousseau's assessments of Hobbes and Grotius converge. The argument against Hobbes that force cannot be lawful blends easily into the argument against Grotius that slavery cannot constitute a right. Thus here in his summary Rousseau can assert that Hobbes' and Grotius' "principles are exactly alike."