Only by South

[lamford]

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In the pub at the North London Club on Tuesday we had the choice of watching highlights of the France v Belgium match or working out what lead would have beaten me as North in Six Diamonds. On the actual 6♠ lead, I was able to squeeze East in spades and hearts in the ending, but the all-knowing all-seeing bot told me that I could have been beaten, but only if played by North. You may enjoy working out how and why without asking the Oracle.

[Cyberyeti]

Obvious what the lead must be as there's only one suit where it makes the slightest difference which side it's led from, why is much more subtle.

[dokoko]

On a high trump lead by West, South wins as follows:

 

Win ♦J with the queen and play three rounds of hearts, ruffing with ♦7.

 

A. West overruffs. Take his spade return in dummy, ruff another heart high, finesse in trumps and cash out for 12 tricks. If instead he returns a club, take in hand, reach dummy in spades and continue as above. If instead he returns a trump, take in dummy as high as necessary, ruff a heart and cash out.

 

B. West discards a spade. Play small to ♠K and play the ♠10 to jack and ace.

---- If West ruffs and returns a club, take in hand, cash the second club and play three rounds of trump squeezing East in the majors.

---- If West ruffs and returns a trump, win in hand, draw the last trump and play three rounds of clubs, finessing the king and ruffing the third. Again East is squeezed.

---- If West discards, play a small trump towards dummy. West must split and you take with ♦A. Now play three rounds of clubs, finessing the king, discarding a spade on the second and ruffing the third. Ruff a heart high and play a spade to make ♦8 en passant.

 

C. West discards a club. Play ♠A and small to ♠K. Play two rounds of clubs, finessing the king and discarding a spade from dummy and play a spade.

---- If West discards, ruff small, ruff a heart high and ruff a club. West can take the last trick.

---- If West ruffs high and returns a trump, win in dummy, ruff the last heart and take the last tricks in dummy.

---- If West ruffs high and returns a club, ruff in dummy, ruff the last heart and finesse in trumps.

 

This line obviously doesn't work when East leads a trump as it involves dummy's ♦A8x sitting over West's ♦JTx (or similar) later. Remains to show that there is no other winning line against East's trump lead and that NS succeed on the lead of any side suit.