(These conditions, naturally, tend to go together.)
In such cases, troops are provided only while remaining under the national command authority of the nation that provides them, though perhaps operating within a command structure headed by a general of some other member state among those providing troops -- generally the largest of such states. In cases of this sort, which characteristic of larger United Nations-authorized interventions, such as the first Gulf War in 1991, the United Nations has no operational authority at all. Viewed from a strictly military perspective, such interventions are indistinguishable from a non-United Nations intervention, as by an ad-hoc coalition.
However, while all of the above points are technically correct, they are too narrowly focused on the purely military aspects of an intervention. The true role of the United Nations is exercised in the all-important political dimension of an intervention. In many respects this political dimension comes most to the fore in the latter stages of an intervention, when the military situation has been brought under control and the process of reconstruction must begin. However, the most important role of the United Nations is one that is felt from the very outset of an intervention: the role of international legitimization of an intervention action. The United Nations Charter provided the Security Council with the authority to "to impose, and r