What was the most important military operation of June 1944? Not Operation Overlord (D-Day), but Operation Bagration, says Mike Davis. The operation began on 19 June 2004, when
…a Soviet guerrilla army emerged from the forests and bogs of Belorussia to launch a bold surprise attack on the mighty Wehrmacht’s rear.
The partisan brigades, including many Jewish fighters and concentration-camp escapees, planted 40,000 demolition charges. They devastated the vital rail lines linking German Army Group Centre to its bases in Poland and Eastern Prussia.
Three days later, on June 22 1944, the third anniversary of Hitler’s invasion of the Soviet Union, Marshal Zhukov gave the order for the main assault on German front lines. Twenty-six thousand heavy guns pulverised German forward positions. The screams of the Katyusha rockets were followed by the roar of 4,000 tanks and the battle cries (in more than 40 languages) of 1.6 million Soviet soldiers. Thus began Operation Bagration, an assault over a 500-mile-long front.
Wikipedia has more.