"Military tensions around Syria shot up again Monday, June 18, with an Iranian report that a joint Russian-Chinese-Iranian exercise is to take place in Syria, called “the biggest of its kind ever staged in the Middle East,” with 90,000 personnel, 400 air planes and 900 tanks taking part." - DEBKA FILE
Provided that this rumor is true, it means roughly a half dozen interesting assumptions.
1) this is unprecedented use of military force to affect international politics on the part of China, meaning another great and official step in its emergence as a world power;
2) this is an unequivocal show of support for the Assad regime from the "red" block;
3) Being a Syrian rebel just went from bad to worse;
4) The US was about to invervene and this is a preemptive troop positioning to counter that (since the US won't want to engage Russian or PLA troops directly), and;
5) Said "red block" is bolder than ever in their antagonism of US policy, putting their troops on the line and therefore national prestige... something the US will undoubtedly fold against for fear of starting a major power war over very little of strategic import.
UPDATE: This troop movement is reportedly a disinformation campaign on the part of Iran. It is unclear what was hoped to be obtained from this action however, as reports like these can be quickly refuted in time even by open source research.