Returning to the 3rd Division, Dannatt planned two exercises at the British Army Training Unit Suffield in Canada. The first was, at the time, the largest exercise the Army had run since the end of the Cold War; the second only took place after Dannatt's tenure as commander had expired. Dannatt gave evidence as an expert witness in the trial of Radislav Krstić in relation to the Srebrenica massacre, shortly after which he was posted to Bosnia, where he served as deputy commander of NATO's Stabilisation Force in 2000. His tour, originally scheduled to last a full year was cut short when Sir Michael Willcocks took early retirement from the Army in order to become Black Rod. The resulting personnel changes to fill the vacancy meant that Dannatt was appointed Assistant Chief of the General Staff (ACGS) in April 2001. In September 2001, he was on a visit to British troops in Cyprus and watched the immediate aftermath of the September 11 attacks on television. As ACGS, he was peripherally involved in planning for the Army's subsequent involvement in Afghanistan and later Iraq, as well as standing in for the Chief of the General Staff (then Michael Walker) when Walker was unavailable. Dannatt was succeeded as ACGS by David Richards, to whom he had handed over command of 4th Armoured Brigade in 1996 and who later succeeded Dannatt as Chief of the General Staff.