Rawls distinguishes between peoples and states. He asserts that states have "the powers of sovereignty" (Law of Peoples, p. 25ff). This is surprising, if ironic, given the position of which he argued in A Theory of Justice where individuals or people seemed to be the originators of the two principles of justice. If states have the powers of sovereignty and states are separable from peoples, then the peoples of a democratic state cannot be said to hold the power of sovereignty. One is left to wonder where or how Rawls' view in Law of Peoples connects with democracy.
With sovereignty, there are two powers that states hold: (1) the right to go to war in pursuit of state policies and (2) a certain autonomy in dealing with the state's own people. Despite that states have these powers, Rawls believes that states should not have these powers. This is an interesting, though confusing, position to take up. If states ought not to have these two powers, then their holding them should not follow.
The view that states should not hold a certain autonomy in dealing with the state's own people is an entirely sensible position. The citizenry of a state should want to hold the government accountable for wrongdoing against the population, such as when the state incarcerates one or more of its citizens for having done something relatively non-offensive. For example, maybe an individual or group has protested a government policy that the group finds draconian or inferior to another option. The group seeks redress but to no avail. When it is uncovered that members of the group protested against the government's policy, the government steps in and arrests members of the group. The government's having the right to deal with their own people is fine, but arresting them for no apparent reason other than that they were voicing their opinion against the government is to be unnecessarily punitive.
States should not have the right to persecute its citizens unjustly. If, however, the person has committed a crime against other citizens, then the state does have a right to punish the offender. The extent of the state's power over its people seems debatable, but it must have the right to punish offenders. So, it is a question of scope how much latitude the government has in dealing with offenders. Some, no doubt, would argue that offenders shouldn't be granted any rights, and the government has no responsibility to ensure that the offender is protected by the rule of law. If this is what we mean by a state's autonomy - which seems to be a narrower definition than Rawls assumes but it is still a definition - then the state has a certain autonomy in dealing with its citizens.
Also, if one of the state's policies is to defend any (or all) of its citizens, then the state has not only a right to go to war in pursuit of the state's policy but it might be seen to have an obligation to do so. Is the obligation what Rawls wants to avoid?