Simon Crean's former national security adviser Carl Ungerer says missile defence could promote international security THE issue of Australian participation in US missile defence plans continues to divide political parties in both countries. Opponents of missile defence, usually from the Left, complain that it is too expensive, operationally uncertain and potentially destabilising. Supporters, mostly from the Right, argue that missile defence is needed because of a gathering long-term threat from countries such as North Korea and Iran. The debate has become polarised and intractable. Later this year, missile defence will be thrust back into the political spotlight with the commissioning of the first ground-based interceptor site at Fort Greely in Alaska.