Establish a national priority to develop fusion energy: end fossil fuels, end climate change, regain world leadership.

For decades researchers have used limited funding to attempt to harness fusion power - the process within the sun - to create virtually limitless energy from (abundant) heavy water, for example, without generating toxic waste of any kind.
The benefit of these power plants to civilization would be revolutionary and sweeping: Elimination of dependence on fossil fuels, Harmony with Earth's ecosystem, Abundance of nearly free energy, Economic world leadership, Elimination of wars for foreign resources.
Given the enormous payoffs of such a technology, the United States should create a national priority to harness fusion power on the scale of Kennedy's space program. Begin with $200B in federal funding for R&D, which would also stimulate the economy, and rebuild US science leadership.