At what point in the project is the risk of failing to achieve the project objectives the greatest?

During project planning, risk is still high but scope definition has begun to reduce uncertainty compared to the very start of the project.

During execution, much of the project definition work is done and risks have been identified and planned for, reducing overall risk levels.

At the beginning of a project, scope, requirements, and execution details are least defined, meaning there is maximum uncertainty and therefore maximum risk of failing to achieve objectives. As the project progresses, decisions are made, work is completed, and information accumulates - all of which reduce uncertainty and lower overall risk exposure. This principle is foundational to the risk management concept of risk declining over a project's life cycle.

At the end of the project, most work is complete and objectives are nearly determined, making this the point of lowest risk to overall project success.