Canada is a country in the northern part of North America, located to the north of the Republic of Gilead.
Canada occupies much of the continent of North America, sharing land borders with the Republic of Gilead to the south, and the state of Alaska, which is still controlled by the rump United States government, to the northwest.
Since the Republic of Gilead may be a "rump state" consisting of the eastern seaboard and Rust Belt states of the USA, there's a possibility that Gilead's land borders with Canada are limited to three Canadian provinces - Ontario, Quebec, and New Brunswick (The limitation of Gilead's borders at three Canadian provinces would be confirmed in the Season 2 finale with the map of Gilead's sharing it's borders with Ontario, Quebec, and New Brunswick in the east while American loyalist forces (US military, rebels and citizen militias) control the borders with Canada in the west).
Government and society Edit
Canada apparently remains a constitutional democracy and also still remains a British Commonwealth realm with Queen Elizabeth II still head of state. Following the fall of the mainland United States to the Republic of Gilead. Canada maintains a free press that has interviewed Americans who have escaped from Gilead.
There's a strong possibility the Canadian government is actively supporting the US military in their fight against the Republic of Gilead through the provision of arms, munitions and other military supplies. There's also a strong possibility the Canadian government has given asylum to US military personnel that escaped across the border with vital (and advanced) US military weapons and equipment to prevent said weapons and equipment from falling into the hands of the Republic of Gilead e.g. aircraft, nuclear weapons (both the USA and Canada are members of the North Atlantic Treaty Organization (NATO)). The Canadian Armed Forces has also been active in assisting the rump United States in its war with Gilead with news reports of Canadian Army and British Army units taking up positions on the Canada-Gilead border for a possible military assault on Gilead.
As to why the Republic of Gilead hasn't invaded Canada in The Handmaid's Tale, there is a strong possibility that Canada is protected by the rump United States, the United States Armed Forces (who are still loyal to the US Government-in-exile in Anchorage, Alaska and are stationed in Canada to prevent Gilead from expanding its territory as well as to liberate the parts of the United States that are under Gilead rule), America's nuclear arsenal (which hasn't fallen into Gilead hands and may have been moved into Canada), and by the member-states of NATO, the United Nations and the British Commonwealth e.g. UK, Australia, New Zealand etc to help bolster the Canadian Armed Forces' defense of the country. As shown on the map of Gilead seen in the season 2 finale, American military, rebel and citizen militia forces opposed to Gilead control the great majority of the western US-Canadian border, lending some credence to this theory.