 Hello, I'm Jackie Leonard, and you're listening to the Newsroom from the BBC World Service. World leaders are in Washington, D.C., where the NATO conference is underway. President Biden described the grouping as the greatest defence alliance in the world at the opening of a summit to mark 75 years since it was founded. And he used his address to promise five more air defence systems for Ukraine. Our North America correspondent, Tom Bateman, listened to the speech.