Complex sentence: During the conflict, the long-standing bitter rivalry between Arasaka and Militech causes them to forget about their customers and go for each other, in the beginning quietly (the phase called the "Shadow War").
Split into simpler sentences: During the conflict, the long-standing bitter rivalry between Arasaka and Militech causes them to forget about their customers and go for each other. In the beginning they feud quietly (the phase called the "Shadow War").