How did the Roman Empire expand into Eastern Europe and the Middle East so much in such little time

When the Roman empire was at war, you can literally watch on the world history animation all these territories being conquered in a few years in Eastern Europe and Middle East. I mean how?? The empires expansion pace was on completely different level than let's say the Mongols or Arabs.<br><br>I'm interested in details, because in my mind that is such an unimaginable feat when I think of it. I mean you have to send thousands of Roman soldiers into places hundreds of km away, cross rivers, organize the logistics, there must have been huge losses along the way. And still they were able to conquer so much territory so fast.