ligny

Wellington rode to Blücher and promised to aid him at Ligny but he didn't which resulted in Prussian defeat at Battle of Ligny.

Whether or not Wellington promised to come to Blücher's aid at Ligny is debatable.

From Prussian point of view 1) they just lost a battle badly at Ligny 2) Wellington said he'd come but he didn't 3) Wellington could escape from sea in which case Prussians will surely be crushed facing French alone.

Trying to translate your last sentence to English gives "Mauldite in itself very all the ligny".

Blücher's Prussian army arrived to save Wellington's ass despite Wellington not saving theirs at Battle of Ligny.

Arguably Napoleon's biggest mistake was not to make sure he had decisively beaten Blücher at Ligny.

Moving from the south-west he defeated Blücher at Ligny, whilst pushing his left wing under Ney to block Wellington to the north at Quatre Bras.

Imagine you are playing Napoleon at Ligny: How much fun is it that your reserves receive conflicting orders all day from a field marshal fighting in a different nearby battlefield, and that there are similar town names in the map, leading to your troops coming in late and in a useless location?

Trying to translate your last sentence to English gives "Mauldite in itself very all the ligny".

Blücher's Prussian army arrived to save Wellington's ass despite Wellington not saving theirs at Battle of Ligny.

Arguably Napoleon's biggest mistake was not to make sure he had decisively beaten Blücher at Ligny.

Moving from the south-west he defeated Blücher at Ligny, whilst pushing his left wing under Ney to block Wellington to the north at Quatre Bras.