The war was started by having a power structure based around a system inheritance where political change and stability is impossible without violent action. Either side's family would have died with Otto and Daemon at the head to secure the inheritance (which they are currently depsite the kneeling and oath swearing to Aegon and Rhaneyra). Viserys chose a weak claimant and everyone is surprised a political member in an advantageous position took control. The problem I always had with the Blacks is they always refused the play the political game and rested on their stale oaths to win the day. Its called a Game of Thrones for a reason swearing oaths is great but anyone with any political sense would have shored up their power through practical means.