The openings at Black's disposal against 1 d4 fall roughly into three categorgies. He can choose the safe but stodgy Queen's Gambit Declined (the QGD is out od favor these days); he can play the sharp but risky "Indian" openings such as the King's Indian, Modern Benoni, Benko Gambit etc.; or he can opt for the family of openinngs which constitutes the middle ground between the above extremes - the Nimzo-Indian, Queen's Indian and Bogo-Indian defenses.

The last group allows Black to fight for the center more actively than does the typical QGD, yet avoids giving White as much central leeway as the second group. Consequently these defenses are the most popular with the trendsetters of opening fashion, the top grandmasters, most of whome regard the pure Indian defenses with suspision. 

The Nimzo-Indian has proven to be an extremely reliable weapon, so many players avoid it by playing 3 Nf3 after 1 d4 Nf6 2 c4 e6. This has brought about a marked increase in the number of QIDs. Once considered dull and drawish, many new and interesting possibilities have been uncovered for both players, of which the renaissance of 4 a3 has been most remarkable.

This move has been known for some time, but was largely ignored till the last few years even though Petrosian had employed it sporadically with good results. Its full potential was realized only after the latest Soviet chess phenomenon, Gari Kasparov, molded it into a respectable system which he has used to win game after game against formiddable opponents. Kasparov's results, and the line's relatively unexplored nature, have been chiefly responsible for its current resurgence.