What is the main conflict in Tomorrow, When the War Began by John Marsden?

Tomorrow, When the War Began by John Marsden is the first book in the Tomorrow Series. It tells the story of Elle, a senior in high school, who has invited her friends to skip Commemoration Day and join her on a camping trip. They agree and venture into a valley known as Hell where they spend a week relaxing. Arriving at their homes, they realize the country has been invaded and their friends and family have either been captured or killed. Realizing a rescue attempt may be futile, they struggle to make sense of these recent events and how to proceed for the future.