It’s entirely realistic if the county felt threatened.
In answer to your question, Britain reintroduced conscription in 1939 before the war broke out.
Australia had national service in the 1950s and again in the 1960, the latter predated the Vietnam War. In neither case was the country directly threatened.
1964 to be exact. In 1951, Eighteen-year-old men were required to undertake 176 days of military training as part of the National Service scheme. Those who elected to undertake their training in the army could break up their training requirements into two periods, 98 days in the Australian Regular Army and 78 days in the Citizen Military Forces (CMF). Those who elected to undertake their training with the Royal Australian Navy or the Royal Australian Air Force had to complete their 176 days in one stretch.
The National Service scheme was discontinued in 1959 because the Australian Government didn't see much use in training conscripts to paint rocks and mow lawn which is all they were employed to do. Conscription takes a lot out of an army as well as adding something to it. Conscription dilutes the manpower as soldiers of the established force as used to train and command the conscription forces. Approximately 25% of the existing Australian Army was employed in the 1960s in training conscripts.
National Service was reintroduced in 1964, and in May 1965 the Coalition Government introduced new powers that enabled it to send national servicemen overseas. At that time Australian soldiers were involved in the war in Vietnam and with Indonesian Confrontation. The Menzies Government wished to raise the army's numbers to 40,000 in order to meet overseas commitments.
All 20-year-old males had to register with the Department of Labour and National Service, and their names were selected by the "birthday ballot", in which men were randomly selected for national service by their date of birth. Those who were selected for national service were required to serve for two years full-time in the Regular Army and three years part-time in the reserves.
Exemptions were given to Aborigines and Torres Strait Islander peoples, the medically unfit, and theology students. Young men were granted exemption on the grounds of conscientious objection if they could prove their objection to war was based on religious beliefs. A temporary deferment of national service was granted to university students, apprentices, married men, and those who could prove that national service would cause them financial hardship. Conscientious objection was hard to prove and several men went to prison as a consequence including Simon Townsend, later a TV Presenter of childrens programming.
If a conscriptee was already enlisted in the CMF they were exempted from conscription. This meant that when the ballots were pulled, most CMF units saw a massive increase in numbers, which then fell away after the danger of being conscripted decreased. The Government reacted by changing the regulations to that now a soldier needed to enlist for six years to be exempted from conscription. A large number of "draft dodgers" then enlisted - my oldest brother amongst them - on the advice from our father who had served in the Northern Territory during WWII and had spent several years trying to be accepted by the AIF but failing because he fulfilled a useful role in the Miltia as a dental mechanic. The pay in the AIF was deliberately higher than the Militia's and that was always handy.
It comes down to how the government of the day and the people are feeling, and again, in this thread, that comes down to the “twitch”.
Which was the point I was making all along...