Q:

Ukraine on Thursday disbanded an army unit that lost six armoured vehicles - an vs. the

Source: http://news.yahoo.com/ukraine-disbands-army-unit-fiasco-push-east-144943083.html

Ukraine on Thursday disbanded an army unit that lost six armoured vehicles to pro-Russian militants, as Kiev's military reeled from a disastrous attempt to oust separatists in its eastern regions.

Why is it an instead of the? They are obviously talking about the unit that was captured by the protesters in Slavyansk a day or so ago. Of course, I understand that the authors know this but they still can speak about what happened in general terms - the unit which was captured was a unit that lost six mechanized vehicles. Okay, they are just getting rid of a unit that was ineffective. Again, what makes you use an instead of the? To me If it was the, it would pretty much convey more or less the same meaning.

A:

I agree with Edwin, that the choice of the article is in in the interests of the reader rather than the writer. It is telling the reader, who may not know about the earlier event, that AN army unit has been disbanded.
Nonetheless I do think the sentence is poorly and ambiguously worded. It could be taken to mean that there was more than one army unit that lost six armoured vehicles, only one of which has been disbanded.
It is also not clear if the final adverbial clause, beginning 'as Kiev's military reeled...', qualifies the verb 'disbanded', or the verb 'lost'. The meaning is slightly different depending on which is the case. For practical purposes it probably refers to the whole scenario of loss and disbandment.
But had I been writing the sentence I think I might have said: 

Ukraine on Thursday disbanded an army unit; the one that lost six
  armoured vehicles to pro-Russian militants. All this happened as
  Kiev's military reeled from a disastrous attempt to oust separatists
  in its eastern region.

