A massive rip in the in the very fabric of reality was caused inadvertently by the Scrins use of their rift weapon. This anomaly would eventually be known has the Dimensional rift. The effects were devastating, the time line was shattered and suddenly people, events, vehicles even buildings suddenly ceased to exist. Suddenly both GDI & Nod had vastly altered armies. Old technology was mixed in with the new, briefly the war was suspended, has both armies struggled to come to terms with both their loses and their new additions. And what of the Scrin? completely unaffected! It was 3 weeks after the dimensional rift that the first skirmish occurred. Nod forces destroyed a relief convoy that was moving to assist a small GDI outpost, all personnel were lost. Belatedly GDI declared war on Nod, the tools of war may have change, but Tiberium was still present, Kane still alive and mankind's future still uncertain.
Tiberian History release 4. A few more historical additions.