The Romanian line was breached at Stalingrad causing the Eastern front for the Germans to collapse.
The Romanian move won’t look super dumb later this year [1940] or early next, when Stalin makes territorial demands.
That will also cause Hitler to send troops to Romania, to protect Ploesti [his major oil source], tie Romania closer to the Reich, and put the seal on Hitler’s decision [if he needed one] to settle up with Stalin.