Final answer:
The formal end to US military combat operations in Iraq and the transition to advising, assisting, and training Iraqi security forces was announced by President Barack Obama in October 2011.
Step-by-step explanation:
The formal end to US military combat operations in Iraq that focused on advising, assisting, and training Iraqi security forces was announced by President Barack Obama. On October 21, 2011, President Obama declared that all U.S. troops and trainers would leave Iraq by the end of the year, effectively bringing the U.S. mission in Iraq to an end. This marked the completion of a gradual withdrawal of combat forces that began in late February 2009, with the remaining troops transitioning to an advisory role.