Why were the opposing forces of World War I locked in a virtual stalemate until 1917? What broke the stalemate?

The main reasons why World War I fell into stalemate on the Western Front are technological.  The technology available at the start of WWI strongly favored the army that was defending.  At the start of WWI, armies had machine guns and artillery and airplanes.  They did not have effective tanks or things like submachine guns.  They did not have much in the way of motorized transport.  This meant that an army that entrenched would be very hard to dislodge indeed.  It would have pre-sited machine gun emplacements that could sweep the approaches to a trench, decimating attackers.  It had artillery that could pound attackers who were out in the open.  It had airplanes that could conduct reconnaissance, which made it much harder to gather a large body of troops for an offensive without the enemy knowing what was happening.  The attackers, at the beginning of the war, had nothing with which to balance these advantages that the defense enjoyed.

By 1918, a variety of factors had changed that allowed the Allies to break the stalemate.  On the tactical level, there were some new weapons and tactics.  Attacking troops could now carry submachine guns, thus giving them more firepower.  They could be supported by tanks, which could give them a way to defeat machine guns.  They improved their tactics, making better use of their own artillery as they advanced.

There were also economic factors that helped the Allies break the stalemate.  The main factor was that the British blockade of Germany was more effective than the German blockade of England.  The German U-Boat menace had largely been broken by this time.  Meanwhile, the British blockade had helped to wear down the German economy.

A final factor came when the US entered the war.  The US troops were not decisive in tactical terms, but they helped tremendously in strategic terms.  The Germans knew that essentially unlimited numbers of American troops would become available over time.  This made them desperate.  It contributed to Germany’s decision to launch the “Spring Offensive,” which did not result in German victory and which cost the Germans dearly in terms of men lost in battle.  The impending participation of the US also sapped German morale.  If the two sides had been throwing everything they had into the war and ending up in a stalemate, it was clear that having a large new force thrown into the balance on one side would make a huge difference.  Because both sides knew this, German morale fell while Allied morale improved.

All of these factors helped to allow the Allies to break the stalemate on the Western Front in 1918.