When the First World War broke out on 4 August 1914, Rodd immediately wanted to enlist – although his mother was not enthusiastic. Three days later, he applied for a commission in the Royal Field Artillery (RFA) through the Officers’ Training Corps at Oxford and was accepted. In the period 4–8 August, over eight thousand men enlisted, while over a hundred thousand had joined up by 22 August. Why exactly Rodd wanted to enlist, after only a year at Balliol, is not clear. But his wish to enlist immediately indicates, if not necessarily support for the war in an abstract sense, a strong wish to be involved in it. Patriotic instincts seem a good explanation for this. But there were probably other factors at work too. It may be that he simply did not want to stand aside while his peers were suffering and dying. An alternative explanation is that he thought war meant adventure; he always wanted to have a full experience of what life had to offer. There is also evidence to suggest that he wanted to enhance his profile among his contemporaries. In March 1915, he wrote to Jeanne Malcolm – daughter of the actress Lily Langtry and a good family friend: ‘It will be a grand thing to come home with a war to your credit especially at my age.’

Rodd's first task was to do a three-week gunnery course at Shoeburyness. This experience exposed him to a different social circle than he was used to. He found a sense of camaraderie there among the new soldiers that remained with him long afterwards: ‘Those days were some of the most wonderful. We who knew nothing about soldiering were set down to train men who knew even less.’ It was a matter of pride for Rodd that he had been among the first one hundred thousand to enlist: ‘We were the first Hundred Thousand and knew it […] Officers and men are now trained under less romantic conditions though they cannot be keener or have more Esprit de Corps than we had.’ Rodd was soon plunged into a position of leadership in 48th Brigade, 14th Division. A few weeks after starting, he reported to his father: ‘So far I have got on very well. I am the sole officer in charge of the ammunition column to the brigade.