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Inspired by a recent question, what is the longest possible sequence before White can make a certain capture? Basically, White is trying to capture a chosen piece, and Black optimally defends that piece from capture. But White will be able to capture it in a finite number of moves. However, the positon must be legal, i.e. it is possible to reach

The longest sequence I have come up with is 156 moves. White wants to capture Black’s bishop.

[FEN "2Bk4/p2P2pp/2pPp3/8/p7/P1p1p1P1/2P1P1Pb/5K1N w - - 0 1"]

1. Ke1 Bg1 2. Kd1 Bh2 3. Kc1 Bg1 4. Kb1 Bh2 5. Ka2 Bg1 6. Ka1 Bh2 7. Kb1 Bg1 8. Kc1 Bh2 9. Kd1 Bg1 10. Ke1 Bh2 11. Kf1 a6 12. Ke1 Bg1 13. Kd1 Bh2 14. Kc1 Bg1  15. Kb1 Bh2 16. Ka2 Bg1 17. Ka1 Bh2 18. Kb1 Bg1 19. Kc1 Bh2 20. Kd1 Bg1 21. Ke1 Bh2 22. Kf1 a5 23. Ke1 Bg1 24. Kd1 Bh2 25. Kc1 Bg1 26. Kb1 Bh2 27. Ka2 Bg1 28. Ka1 Bh2 29. Kb1 Bg1 30. Kc1 Bh2 31. Kd1 Bg1 32. Ke1 Bh2 33. Kf1 c5 34. Ke1 Bg1  35. Kd1 Bh2 36. Kc1 Bg1 37. Kb1 Bh2 38. Ka2 Bg1 39. Ka1 Bh2 40. Kb1 Bg1 41. Kc1 Bh2 42. Kd1 Bg1 43. Ke1 Bh2 44. Kf1 c4 45. Ke1 Bg1 46. Kd1 Bh2 47. Kc1 Bg1 48. Kb1 Bh2 49. Ka2 Bg1 50. Ka1 Bh2 51. Kb1 Bg1 52. Kc1 Bh2 53. Kd1 Bg1 54. Ke1 Bh2 55. Kf1 e5 56. Ke1 Bg1 57. Kd1 Bh2 58. Kc1 Bg1 59. Kb1 Bh2 60. Ka2 Bg1 61. Ka1 Bh2 62. Kb1 Bg1 63. Kc1 Bh2 64. Kd1 Bg1 65. Ke1 Bh2 66. Kf1 e4 67. Ke1 Bg1 68. Kd1 Bh2 69. Kc1 Bg1 70. Kb1 Bh2 71. Ka2 Bg1 72. Ka1 Bh2 73. Kb1 Bg1 74. Kc1 Bh2 75. Kd1 Bg1 76. Ke1 Bh2 77. Kf1 g6 78. Ke1 Bg1 79. Kd1 Bh2 80. Kc1 Bg1 81. Kb1 Bh2 82. Ka2 Bg1 83. Ka1 Bh2 84. Kb1 Bg1 85. Kc1 Bh2 86. Kd1 Bg1 87. Ke1 Bh2 88. Kf1 g5 89. Ke1 Bg1 90. Kd1 Bh2 91. Kc1 Bg1 92. Kb1 Bh2 93. Ka2 Bg1 94. Ka1 Bh2 95. Kb1 Bg1 96. Kc1 Bh2 97. Kd1 Bg1 98. Ke1 Bh2 99. Kf1 g4 100. Ke1 Bg1 101. Kd1 Bh2 102. Kc1 Bg1 103. Kb1 Bh2 104. Ka2 Bg1 105. Ka1 Bh2 106. Kb1 Bg1 107. Kc1 Bh2 108. Kd1 Bg1 109. Ke1 Bh2 110. Kf1 h6 111. Ke1 Bg1 112. Kd1 Bh2 113. Kc1 Bg1 114. Kb1 Bh2 115. Ka2 Bg1 116. Ka1 Bh2 117. Kb1 Bg1 118. Kc1 Bh2 119. Kd1 Bg1 120. Ke1 Bh2 121. Kf1 h5 122. Ke1 Bg1 123. Kd1 Bh2 124. Kc1 Bg1 125. Kb1 Bh2  126. Ka2 Bg1 127. Ka1 Bh2 128. Kb1 Bg1 129. Kc1 Bh2 130. Kd1 Bg1 131. Ke1 Bh2  132. Kf1 h4 133. Ke1 Bg1 134. Kd1 Bh2 135. Kc1 Bg1 136. Kb1 Bh2 137. Ka2 Bg1 138. Ka1 Bh2 139. Kb1 Bg1 140. Kc1 Bh2 141. Kd1 Bg1 142. Ke1 Bh2 143. Kf1 h3 144. gxh3 gxh3 145. Ke1 Bg1 146. Kd1 Bh2 147. Kc1 Bg1 148. Kb1 Bh2 149. Ka2 Bg1 150. Ka1 Bh2 151. Kb1 Bg1 152. Kc1 Bh2 153. Kd1 Bg1 154. Ke1 Bh2 155. Kf1 Bg1 156. Kxg1

Dows anyone know of a longer such sequence, or possibly create a longer one?

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Fun concept! I think you can at least retract a couple of moves from your extended example: R: 1. a5-a4 a2-a3 2. a6-a5 fxRg3 3. fxNe6 Nf8xRe6+

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