Dividing a country

Let's say if I wanted to divide France in 2 halves. How would I do this? Do I need to go around half of the borders again, basically having double borders where the two halves meet? I can't figure it out.

Basically, can you a use part of an existing country's border to form a new country or do you have to redo the whole part?

Can anyone help?

If I remember you can just select a point, then press delete. Do this with every curving point you have in surplus. And you should get a place to create a new country. Now just create a new one following all the boundarys that suround this country.