Complex sentence: After Dong moved the capital to Chang'an, Guo was sent to assist Niu Fu in the defense of Mei, and because he stationed outside the capital, he was able to return to Liang Province when Dong Zhuo was assassinated by Lü Bu.
Split into simpler sentences: After Dong moved the capital to Chang'an, Guo was sent to assist Niu Fu in the defense of Mei. Due to the fact that he stationed outside the capital, he was able to return to Liang Province when Dong Zhuo was assassinated by Lü Bu.