DoW Alive and Well
Actually I would posit that declarations of war very much remain a legal and accepted method of initiating hostilities between states in the nuclear era.
The Korean war is certainly an example of declarations of war being used, even though one did not precede the beginning of the conflict due to the need for surprise, and in fact there still exists a state of war between the two korean states.
Again we see formal states of war existing between the Arab Israeli states in 1956, 1967 and 1973. The franco-british move to the suez in 1956 was carried out with subterfuge, claiming to be acting as 'peace-enforcers' and that actuall foreshadows several western actions later on that also try to generate legitimacy through unconventional means.
Vietnam was another setting for a traditional war although with the added element of an insurgency in south vietnam and of course american involvement and participation through their south-vietnamese proxy.
The great difference appears to be that with larger, and more effective, standing armies the need for prolonged mobilisation is much decreased and therefore a formal declaration of war prior to the commencement of hostile action could severely compromise the success of the operation.
The modern balkan wars remain an altogether more complex issue. The actual fighting between local forces has clearly been a civil war (with foreign sponsors in many cases) but NATO's action has not been 'war' in the traditional sense because being aware of the dubious legality of their actions (especially in Kosovo) the allies justified their operations with reference to the UN charter, while purposefully ignoring those sections of the charter which directly countermanded their operations.
This could turn into quite a fun topic... and I hope all of you will bear with my comments being a little rambling in nature, but I didn't feel like breaking out the library at this point.