can someone explain the beggining of the NOD campaing on Tiberian Sun?
starts off really confusing.do I need to play Tiberian Dawn to understand?

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The plot is a follow up to the original game Command & Conquer, after the end of which the Nod leader Kane is presumed dead. In Tiberian Sun, Kane resurfaces from his hideout with improved military forces and new Tiberium-enhanced technologies, determined to rule the world at all cost. The GDI Commander Michael McNeil is tasked with objectives to prevent the world from falling into the hands of Nod, this time with the very means of the extraterrestrial Tiberium substance.

I don't really know how it's confusing, but then again I know the lore.

Maybe you started the Firestorm campaign first? Did it begin with a news cast with "Today's Execution" or 5 guys on monitor things talking?

If it's the Today's Execution one, I can tell that the Slavik character isn't in the earlier game. They're introducing the cast of "loyal NOD guys" with that daring rescue thing. CABAL is new as well. I think that basically all you need to know is that Kane is sort of a big deal to NOD people, and at the beginning of Tiberian Sun he is presumed dead.

/edit: forgot to mention that the 5 guys on monitor things is the Firestorm campaign, and starting with that would be seriously confusing. :p