Even though Romania came out of World War I with more gains than most other participants, it was not immune from the political instability that plagued many European countries during that period with many governments rising and falling in rapid succession and the emergence of several ultranationalist parties, the most notorious and violent of which being Corneliu Zelea Codreanu's Iron Guard and by the beginning of World War II Romania was a pseudo-fascist royal dictatorship.
Your challenge is to keep Romania a functioning liberal constitutional monarchy by the beginning of World War II, without the constant change in government, with much less political violance and with the fascist movements like the Iron Guard being much less influential, with a PoD starting in 1919.