>>510188809Since you're having a little trouble understanding I'll break it down for you.
Prior to both wars Canada maintained a very small standing army (mainly shell formations used for training) and a large reserve force. The reserve force was essentially a self-armed militia that was used to defend the country and deter invasion from the US. Laws at the time forbade the Reserve from being sent overseas, so it was exclusively a domestic security force.
When war broke out, most of the reserve volunteered to transition to full military service to be sent overseas. This left the reserve force chronically undermanned, as new volunteers volunteered to serve overseas rather than being reservists. Near the end of WW1, several high-profile incidents of sabotage led to paranoia about enemy infiltrators and the government enacted conscription to create a domestic security force to replace the reservists, because the reserve had all volunteered for overseas deployment. This conscription drive mostly took in the elderly, infirm and those ineligible for voluntary service to bolster domestic security and free up the last of the reserve for voluntary deployment, but it was controversial because people feared it may be the slippery slope towards deploying conscripts overseas.
In WW2 the same thing happened, except at the time the fear was not infiltrators, but that the Japanese may attempt to jump off from the Aleutian Islands and invade the pacific coast. So once again, the reserve, which was mostly depleted from reservists volunteering as fulltime soldiers, was replaced by a conscripted domestic security force, mostly formed from those ineligible for fulltime service due to age or preexisting conditions.
In both cases these measures were necessary because so much of the military-eligible population had volunteered for overseas deployment that the government needed to literally force people to stay in Canada to defend it.