Romania isn't going to declare war - it'll just move troops into Moldova's territory - Russia will oblige by attacking those troops without declaring war. All it takes is one Russian missile to land on Romanian troops in Moldova - or to hit Romanian troops whilst being aimed at Moldovian troops - and Romania can invoke the NATO treaty. An attack on Romanian troops in Romania would oblige NATO to respond militarily, but any attack on Romanian troops will at least force NATO leaders to "consult", so even then Russia cannot rely on a complete lack of response from NATO.
Basically - the Romanian-Moldovian pact is designed to make it that much harder for Russia to do in Transnistria what it did in Crimea.
More to the point - your have your example backwards - you said the US was no the aggressor, but according to your logic it was because it attacked Germany, which had not attacked the US, due to Germany's attack on France and Britain. Romania won't be attacking Russia or crossing into Russian territory - Transnistria is in Moldova.
Ah - but this is foreign relations, not politics.
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