Germany
The Geneva Protocol of 1925 banned the use of gas weapons including mustard types. It was further banned by the Chemical Weapons Convention of 1993.
The Germans used mustard gas during world war one.
Although I was unable to find a definite answer after reading the article on the battle, there were a few references to mustard gas (Sulfur Mustard), so that would be my guess.
What makes mustard a "pub mustard" is the fact that honey and possibly Irish whiskey is added to wholegrain mustard. Pub mustard can be used on sandwiches and can also be used in dips.
Mustard was first used as a condiment (and given its name) by the Romans, more than 1,500 years ago, so it could be said that all mustard originated in Europe.
the battle of brain
what is mustard seed used for, what dishes? It's used in marinating chicken.
The Battle of Maxia
It's said that Mustard Gas was used for the first time. Grenades,Mines and Submarines as well. There was also Tanks, but because of all the mud, they would sink quickly.
Strictly, 'the army lost its first battle', or acceptably, 'the army lost their first battle'. The singular is traditionally used about the enemy, though plural is also used, equally correctly.
There are some slight differences, but they are fairly similar.