I would say that the answer to this is “yes.” In my view, war does both of these things. In this answer, I will lay out reasons why I think war does both of these things. See if you think one of the arguments is stronger than the other.
War can clearly threaten freedom. There are often times when countries that are less free attack countries that have more freedom. If the invading country wins, the invaded country will have less freedom. This happened when Germany invaded France in WWII. It happened when Japan invaded the Philippines (the Philippines was a US colony, but enjoyed more freedom under American rule than under Japanese). War can also threaten freedoms on the home front. Governments become so invested in the outcome of the war that they worry that domestic opponents of the war will become a threat. This happened in the US, for example, when it created the Espionage Acts during WWI. In these ways, we can argue that war threatens freedoms.
War can also protect freedom. Wars can be fought for noble causes. While the American Civil War did not start as a war to destroy slavery, it did end up doing so. That war clearly improved the levels of freedom experienced by African Americans. The same is true of WWII, which improved (and this is an understatement) the amount of freedom enjoyed by Jews in Europe. Wars can even improve freedoms in the losing countries by destroying oppressive regimes. This clearly happened when WWII destroyed the Nazi regime in Germany and the militarist regime in Japan.
In these ways, war can threaten or protect freedom. I do not think either argument is stronger so I do not think I can say war does one or the other. Do you find one of these arguments more compelling than the other?