The Treaty of Versailles
The Treaty of Versailles angered the German people for many reasons. The first was that it was a Diktat ('Dictated Peace'): Germany had no say in the formulation of the treaty and was forced to agree to the terms of the treaty regardless of whether its people liked them or not. Over time, other European powers came to realise that the original terms had been too strict. When Hitler broke the treaty many years later, the reticence of the other powers to react immediately was taken as an indication that many of them agreed with Hitler that the treaty had been implemented wrongly anyway. In effect, the Treaty of Versailles didn't settle any disputes; it created more issues between countries already fractious and struggling to recover from the last war.
A significant sticking point was a clause in the treaty that made provisions for the Rhineland (which borders France and Belgium)to become a demilitarised zone - Germany would be prohibited from stationing any military troops in the Rhineland. Additionally, Germany was ordered to disarm, supposedly the first step towards world disarmament, but no one else was encouraged to join Germany in disarmament. Germany would technically be defenceless should France decide to invade them - which they eventually did in 1923 when Germany was unable to pay the reparations owed to France that year.
Reparations - effectively a 'fine' for the damage done by Germany in 1914-18 - was possibly the most unpopular part of the Treaty of Versailles. At the end of the war, land had been destroyed all over Europe, but especially in France. The treaty said that Germany had to make reparations by paying regular sums of money to France. What this clause failed to recognise was that the war had equally damaged Germany's economy and they simply could not afford to pay. The German solution to this was, in retrospect, foolish - they printed more money. The effect was that the German currency devalued to the point where the economy was on the verge of total collapse. This issue of reparations was eventually solved by the Dawes Plan and Germany were able to pay France again.
The Dawes Plan
The Dawes Plan was created so that Germany could afford their reparations to France, but it also seemed to solve many of the world's money problems. America loaned money to Germany, Germany used it to pay for the reparations, France used it to pay Britain the money they owed, and Britain used it to pay America the money they owed. This seemed to be the perfect solution to everybody's problems, and countries started getting along with each other again. However, it had one major flaw - if anything went wrong, and one of the countries was no longer able to pay another, then conflict would ensue and the economic and diplomatic situation could be worse than it was before. This happened in 1929 with the Great Depression.
The Great Depression
The stock market crash in America caused economic strife throughout the world. America could no longer loan Germany money for reparations, and they even wanted the money back. The result was that America went into isolation with the intent of nursing their own economy and avoiding being dragged into another costly European war. Countries all over the world were facing economic crises, and distrust started to form again between countries. Unemployment was high all over the world, and countries solved this problem by creating large armies. The global depression was therefore a contributing cause of the Second World War as it gave Germany an excuse to break the Treaty of Versailles and establish larger armed forces on their own turf.