As a Romanian we don't see our entry into WW1 as a mistake. It's considered a very wise move, since it garnered us a LOT of good will among the Entente and directly led to the unification of almost every single Romanian majority territory that existed outside the borders of the Old Kingdom of Romania. Post WW1 Romania was basically 2x the size and population of pre-WW1 Romania.

What really fucked us up was the collapse of the Russian Empire, since our war plans massively relied on them as the main force on the Eastern Front.