Shogi Tsume Puzzle: March of the Pawn

There are several variants of this sequence, but the following line is the crucial one:

1. 1一飛成
2. 同　玉
3. 2三桂
4. 1二玉
5. 1三歩不成
6. 2一玉
7. 2二歩
8. 同　玉
9. 3三金
10. 2一玉
11. 2二香

The key to this puzzle lies in the fifth move: 1-3 Pawn Unpromoted (1三歩不成). In Shogi, promoting a pawn is almost always advantageous. Cases where not promoting the pawn is the optimal—and necessary—move are extremely rare, making this sequence a brilliant exception.