Brief History

Most Muslim/Arabian armies are successful in conquering lands have led quite a few explanations; in my opinion, I strongly believe that point of time in circa 400 AD, the Byzantines were in war with the Persians due to diplomatic relations as these 2 Empires are the strongest at time. Unfortunately, the war raged for time, and when the 2 Empires got weaker and suffered fatigue due to long periods of wars, it was definitely a good time to for the Muslim Arabs to conquer the lands. So put to put things simply, The Arabs conquered the lands easily since the two strongest Empires got a lot weaker fighting their own wars - Ultimately, they were fighting with weak enemies.

The Arabians are particularly known for their way of building structures that are mainly catered for their hideouts against enemies. So technically, that might be one reason why they won many wars - The tactics of Retreating from enemies.

Castles and Fortresses

Their structures are built to be rock solid; possibly with all external defense walls at a thick thickness, these walls do not shatter easily from enemies trying to destroy it. Also in their many fortresses and hideouts, they've built long spiral staircases that lead them all the way deep underground, secret tunnels and exits for escape, retreating from their enemies and carefully designed trap doors were built that tricks the enemies into locking themselves in. In this case, their type of architecture was specifically designed for military and escape use.

Today

Even till today in the 21st century, I believe the Arabs still continue for a conquest; not a conquest for land or World domination, but a conquest of building a great city - Dubai, UAE (United Arab Emirates)

Well it is all true, the fact they no longer build strong fortresses and powerful armies, but now for a conquest of great architecture, planning and willed with a strong mindset for Dubai to be the fuel of the World's economic power. They have possibly built one of the World's finest hotels, the Burj Al Arab, which can be easily identified by anyone shaped like a sail. Perhaps one of the greatest advancements of architecture is the "soon to be" World's Tallest Building, the Burj Dubai. Though its height being kept a secret, the architects (Skidmore Owings & Merrill) were inspired by the base shape of the desert rose - but overall, it is a sure thing that the Arabs till today are doing what they can to make Dubai a tourist mark on the map.