Major progress in magnetic fusion research has led to ITER - a multi-billion dollar burning plasma experiment supported by seven governments (EU, Japan, US, China, Korea, Russia, and India) representing over half of the world's population. Currently under construction in Cadarache, France, it is designed to produce 500 million Watts of heat from fusion reactions for over 400 seconds with gain exceeding 10 - thereby demonstrating the scientific and technical feasibility of magnetic fusion energy. Strong research and development programs are needed to harvest the scientific information from ITER to help design a future demonstration power plant with a gain of 25. Advanced computations at the petascale and beyond in tandem with experiment and theory are essential for acquiring the scientific understanding needed to develop whole device integrated predictive models with high physics fidelity. This is the primary motivation for the Fusion Simulation Program (FSP) - a new DOE-SC initiative supported by the Offices of Fusion Energy Science and Advanced Scientific Computing Research -- that is entering the project definition phase. Since ITER and leadership class computing are prominent missions of the DOE today, producing such a world-leading predictive capability for fusion represents a key exascale-relevant strategic project for the future.