In the days of antiquity, the Vale of Tempe was held by the Thessalian army intent on stopping a young Macedonian named Alexander from becoming the head of the League of Corinth. This steeply sided gorge, which led west from the Aegean to the interior of Greece, would be difficult for any army to force open. Not to be deterred, the young leader (soon to become Alexander the Great) had his soldiers carve steps into the mountain so he could circumvent the defenders and emerge behind them on the plains of Thessaly. The rest is history.
In 1941, Greek mountains and rivers would not be a deterrent to the Germans either. The British and ANZAC troops fighting in Greece soon found themselves desperately parrying the thrusts of panzer troops, mountain troops, and parachute troops. That they escaped at all demonstrates convincingly the skill, bravery, and determination with which the allied forces fought.
The Invasion Begins
On April 6, 1941, two pincer arms of the German army drove south from Bulgaria into eastern Greece capturing the key locations and piercing the Metaxis line. An additional thrust, first into Yugoslavia and then down into Greece, 
encircled the Greek 2nd Army and forced its surrender on April 9th. All of these thrusts crossed cold snowy mountains - formidable terrain which, to the shock of the Greeks and British, appeared to be no impediment to the German war machine.
In a similarly surprising and dangerous flanking move, Germans crossed into Yugoslavia all the way to Skopje before striking south into Greece through the undefended Monestir Gap. In no time German troops drove the Wilson Force troops positioned at Vermion back across the Aliakmon River.
German forces also sped westwards to cut off the belated retreat of the Greek 1st Army in the Pindus mountains. They stood athwart this line of retreat by April 19th and forced the surrender of the last remaining Greek forces.
Now the British were alone and fighting for time to effect an orderly withdrawal and evacuation. 
For the British and Commonwealth to escape destruction, the vital pass between Mount Olympus and the Aegean was the key. Positioned here were the New Zealand troops, who also covered the strategic Vale of Tempe - the same pass held against Alexander - which lead to the Plains of Thessaly and the city of Larisa, the major route for the withdrawing British-led forces. If these crossroads fell too soon, Wilson Force would be cut off and utterly defeated.
The Fight Along the Coast and for the Vale of Tempe
New Zealand forces at Platamon had been told not to expect any German armor, and so they had not been given much in the way of anti-armor weapons. But, on the night of April 15th, German armor began attempting to break through this position and for two days the fight raged. More than 100 German tanks came up against the meager forces of the New Zealand 21st Battalion, 4 25-pounder anti-tank guns, and a platoon of engineers. Eventually the New Zealanders gave way, and pulled back across the Pinios River. The NZ troops destroyed a rail tunnel along the coast that would have permitted German armor to quickly pursue. They then escaped through the gorge taking up a defensive position behind a destroyed bridge on the western end.
The German 6th Mountain Division took up the chase. Just like Alexander had done 24 centuries earlier, the Germans avoided the pass and came over the mountains. But, as they came down on the north side of the river across from the Allied positions they discovered that the crossings had been destroyed.
Only by using rafts, the German troops forced the crossing of the Pinios, annihilating the entire New Zealand Bn trying to hold the door shut.
The Last Stand
There was little else for Wilson Force to do, but to fall back to the final defensive line and buy time for the forces evacuating from the beaches. This line would be at Thermopylae. In ancient times, the Thermopylae Pass proved to be a decisive defensive position. T
he British hoped this area - a narrow shoreline defile between the sea and high rugged mountains - could help them fend off the Germans long enough to evacuate most of Wilson Force.
At Thermopylae British, Australian, and NZ troops dug in and fought hard, resisting strong enemy attacks. German tactical air support swarmed above, completely unopposed like so many "angry bees." After failing to break the allied position head-on, the Germans again sent mountain troops through the rugged terrain above the pass, and another outflanking maneuver through Molos, forcing the British to abandon the position.
On April 25th, the Germans employed parachute forces landed at Corinth to seize the canal crossing and prevent British troops from escaping. This attack, though successful, came late as most of the British troops had already been evacuated. This "vertical envelopment" hastened the British departure from Greece and foreshadowed the attack on Crete.
Unforseen Consequences
The campaign was a decisive and rather shameful defeat for the British, the gallantry and supreme tactical abilities of Wilson Force aside. It's rather fortunate that, despite total Germany ownership of the skies above the evacuation area, the British Navy was able to rescue 80 percent of the Wilson Force and evacuate them to Crete for rest and re-fitting. They would arrive with barely any of their heavy equipment and in rather short supply of weapons and ammunition.
Very early on, Churchill had warned Wavell to not commit to a Greek campaign if he feared it would just be "another Norway." In 1940, Norway had proved to be a stinging defeat at the hands of the Germans. After the war, Churchill would say of Greece - "This was Norway all over again, and on ten times the scale."
Even worse, the expedition to Greece had devastating consequences for the Allied forces in North Africa. As the two divisions and armor were stripped away from Libya, Rommel was in the process of unloading his panzers at night, under lights on the docks of Tripoli. When he attacked, the troops were sorely missed, as were the trucks and fighter planes that would have been there to support them during Rommel's first stunning attack that devastated the British.
