Winkelman campaigned for the position of Chief of Staff of the Dutch Army, but lost to General Izaak Reijnders.

The Dutch mobilised their armed forces on 28 August 1939, four days before Nazi Germany's invasion of Poland.

Chief of Staff, General Reijnders, was appointed as Supreme Commander of the Dutch forces, but it was clear from the outset that his personal and professional relationship with Defence secretary Adriaan Dijxhoorn left a lot to be desired, ultimately leading to Reijnders' (honorable) discharge on 5 February 1940.

The Grebbe Line was to be defended until the bitter end, as the eastern front of Fortress Holland (the New Dutch Water Line, once the pride of the Dutch defence system was deemed obsolete and too close to major cities such as Utrecht and Amsterdam).

Hitler's bold plan to drop paratroopers around The Hague, push into the city and capture the Dutch government, the Royal Family and the Supreme Army Command to force the Netherlands to its knees within 24 hours, ended in failure.

After one day of war, General Winkelman was relatively satisfied about the way his troops had reacted to the first German push.

Meanwhile, strong German infantry (supported by the 9th Panzer Division) had smashed through the so-called Peel-Raam Stelling and now marched rapidly through the southern province of North Brabant, threatening to establish contact with the bridge head at Moerdijk and to enter Fortress Holland from the south, effectively isolating the Netherlands from Belgium and France.

On 13 May, after the departure of Queen Wilhelmina to London, and with most ministers in Hoek van Holland ready to depart, minister Max Steenberghe, on his own initiative, but in name of the queen and cabinet, granted the powers of government within the European part of the Netherlands to Winkelman, and requested that the permanent secretaries follow his directions.

The bombing of Rotterdam followed and with, the Germans threatening to give major Dutch city Utrecht the same treatment, General Winkelman was forced to surrender in the evening of 14 May.

After he signed the Dutch surrender, General Winkelman refused to officially declare that he would not resist the German forces in the Netherlands.