War became illegal in 1928 with the Paris Peace Pact that created a new world order, according to the lawyer and academic Oona Hathaway. She tells Andrew Marr how this pivotal moment launched a new international system in which sanctions replaced gunboat diplomacy. Although inter-state wars have fallen since World War Two, intra-state conflicts have risen: Elisabeth Kendall explains the dire situation of one of the Arab's poorest countries, Yemen. The Norwegian ambassador to the UK Mona Juul talks about the art of international diplomacy. She played a key role in the secret talks which led to the Oslo Accords in the 1990s - the first ever agreement between Israel and the Palestine Liberation Organisation. And Phillip Collins looks at the speeches that have shaped the world and inspired generations, especially at moments of war and conflict. Producer: Katy Hickman.