On 13 October 1939, Campbell joined the Second Australian Imperial Force (AIF), and was given the AIF service number VX21. He was initially assigned to the &lt;a href="2/1st%20Battalion%20%28Australia%29"&gt;2/1st Battalion&lt;/a&gt; as a major, but became the brigade major of the 16th Brigade that same day. He was made a Companion of the Distinguished Service Order for his part in the Battle of Bardia. His citation read: