The Moldavians were with the rest of the Romanians in hard times, but not vice versa (1918 vs. 1940)

In 1918, when Bessarabia united with Romania, Romania was woefully alone. Wallachia (including Bucharest) was under German military occupation, Dobrogea under Bulgarian occupation, and the Romanian government had taken refuge in Iasi. Also in historical Moldova, the Bolshevized Russian troops looted and terrorized. Practically, the Romanian state was on the verge of dissolution.

And, under these conditions, Bessarabia was the first province to unite with Romania (unification tolerated by the Central Powers, who thus wanted to make the WW1 loss, which seemed obvious at the time, more digestible for the Romanians). Also, through this unification, Romania remained with access to the Black Sea (which it would later lose, following the loss of Dobrogea).

Instead, in 1940, Romania shamefully withdrew from Bessarabia and Northern Bucovina – then also from Northern Transylvania and Quadrilater. The argument since then was "if we fight, Russia will completely occupy us" – but guess what, this happened. Romania abandoned millions of its citizens in the clutches of the Russians and the Hungarian fascists.