I once tried with Poland, and it did not end well.
I agree with Faustling that the problem is the very long border to defend, which becomes only longer when Germany puppets Slovakia. Another problem is that the terrain is not very defensible, mainly plains. Third problem is the German attack from the north. If you could bet them off and capture the German provinces there, you could have marginal possibility of survival. Also it's good to remember that soldiers in HoI3 are too dumb to dig-in during peacetime.
Wish you luck.