At the beginning of the US Civil War, the "southern" border States were Kentucky and Missouri. In this answer, Maryland is excluded in that although it was a pro=southern slave State, it cannot be considered a border State.
Mexico is the country on the southern border of the United States.
There are twelve (12) states which share their border with Canada's southern border. They are: Maine, New Hampshire, Vermont, New York, Pennsylvania, Ohio, Michigan, Minnesota, North Dakota, Montana, Idaho and Washington.
The three states that border New Hampshire are Maine to the southeast, Massachusetts to the south, and Vermont to the west. These neighboring states contribute to New Hampshire's geographic diversity and its access to various recreational and cultural opportunities.
Canada does not have states. Canada has provinces. The Canadian province of Québec shares a border with the American states of New York, Vermont, New Hampshire and Maine.
Three states border Canada and the Atlantic: Maine, New Hampshire, and New York.
Texas and Arkansas each border Louisiana. All three can be described as as "southern States".
New York, New Hampshire, and Massachusetts.
Vermont, Maine and Massachusetts.
Massachusetts.
New Hampshire, Massachusetts, and New York.
New York, New Hampshire, and Massachusetts