Sounds a bit like I'm doing your homework, but you wouldn't be getting reddit to write your homework for you because that would be to piss on the graves of hundreds of thousands of dead soldiers who were killed in one of the most miserable and ill-conceived battles ever fought.

Passchendaele was one part of a larger battle known as the Third Battle of Ypres. Douglas Haig, as you will know the commander of the British Army in France, was under pressure in 1917 for a number of reasons. His attempt to break through the German lines in 1916 on the Somme had been a failure, but the Germans had retreated during the winter following to what the Allies called the 'Hindenburg Line', a system of defences built on favourable ground and fortified as appropriate. French efforts to attack this had failed early in 1917 to such an extent that the French Army fell, in part, into mutiny, and could no longer be relied upon to defeat Germany.

A supporting British battle at Arras, largely fought by Canadians, was partially successful in securing high ground, but any notion of a decisive breakthrough was crushed. On top of this, Britain itself was losing vast amounts of shipping to U-boats, and British towns were being bombed from the air regularly.

Haig decided that he was going to sweep through the Germans in Flanders, the borderland of France and Belgium near the ocean. Usual formula of massive artillery bombardment, with some newer things like some mines (tunnel under the enemy front line, pack in a load of explosives, detonate just before the attack- used at Arras/Vimy Ridge), tanks (used first on the Somme and poorly at Arras), and he had lots of new soldiers available. He had the idea that he could break through and capture the ports in Belgium, which were important U-boat bases, and also capture some German bomber bases.

Flanders is boggy. Having spent some time there I can tell you that even in summer, you don't take your car off the road because even what looks like a grassy verge can be liquid underneath. The whole area is only just above sea level. Most of the German strong points were actually built above ground, as concrete blockhouses, rather than the more-traditional trench and bunker.

Third Ypres opened with a week of rain, and it was a battle fought in pure, liquid mud. Wounded men often drowned in the mud. Moving supplies was a nightmare. Everything became covered in it, and it could almost prevent men moving. Of course, any trench or defensive diggings quickly filled with it. Most tank attacks lost most of their vehicles in it.

Like many Western Front battles before, Passchendaele devolved into a series of slugging matches where soldiers took smashed villages at enormous cost. Eventually exhaustion, the cold of winter, and a lack of replacements saw the battle wind down.

Regarded as the most pointless, horrific and indefensible of Haig's battles, interestingly Third Ypres was the last major example of these characteristic trench warfare slogging matches of the Western Front. Although nobody could foresee it at the time, in November 1917 the war had but one year to run, and almost every subsequent battle involved movement and maneuver.

Sources- well, been studying this stuff for 30 years but Terraine, AJP Taylor, Denis Winter, the various defenders of Haig who have written biographies, Les Carlyon, and thee excellent 'Battleground Europe' series might help.

A good, very detailed answer has already been written. It's called Passchendaele: A New History by Nick Lloyd. I think your local library will have it.

Or is that too detailed?