Complex sentence: By the time the full extent of the Zulu threat had become apparent, it was too late for Pulleine to alter his dispositions, which had left his companies strung out on a long line, acutely vulnerable to being outflanked by such a large enemy force.
Split into simpler sentences: By the time the full extent of the Zulu threat had become apparent, it was largely too late for Pulleine to alter his dispositions significantly. The result was an overextended line, vulnerable to being outflanked by such a large enemy force.