Simple. Mountains. Most of Switzerland sits on the Swiss Plateau which is surrounded by Jura Mountains and the Swiss Alps. When Germany invaded Belgium in WW1, its plan relied upon the mostly flat landscape of Western Europe that would facilitate a quick war, not the dense mountainous landscape of Switzerland. After the Franco-Prussian war of 1870 (where the Germans took much of Alsace-Lorraine from France), the Germans continually anticipated another war with the French. However, they knew there was an emerging network of alliances in Europe that meant if they ended up in a war with any European power, they would probably end up in a war of two fronts (as France and Russia were on the same side, and Russia was against Germany's ally Austria-Hungary). To try and mediate this, the German high command consequently came up with a plan known as the "Schlieffen Plan". Thinking that Russia was more backwards than Western Europe, they decided that in a scenario of fighting both countries, they would have a window of opportunity to take out France while Russia mobilised. Because of the Franco-Prussian war, the French had built a series of fortifications along the Franco-German border to dissuade another German invasion. The Germans therefore decided their best bet for engaging with the French swiftly was through going through neutral Belgium first and attacking the French from the North. This tactic had two benefits: it avoided the Franco-German border, where the Germans thought the French would assume they would mount an invasion, and Belgium is relatively flat allowing quick movement. However, when war actually broke out in 1914, the series of alliances meant that France and Britain both came in on the side of neutral Belgium, and the Germans weren't able to mobilise as quickly as they wanted. This meant they got stuck in trench warfare with the British and French in Belgium and parts of Northern France, and with the Russians on the eastern front. Given that Switzerland had declared its neutrality, neither side would have bothered spending more attacking a neutral country with difficult terrain when they were lodged in a difficult trench war partly sparked by Germany infringing upon a neutral country.

WW2 was different, as Hitler obviously had ambitions to conquer Europe. He also had an especial hatred of Switzerland - although there was a significant German speaking population in Switzerland, it was a decentralised state that also included speakers of French, Italian, Romansch, and other minority languages, and was therefore the opposite of Hitler's dream of a pan-Germanic centralised nation. He once even referred to Switzerland as the "pimple" on the face of Europe, and viewed the country as a the "prodigal son" of the German communities of Europe.

Switzerland lies between Germany in the north and Italy in the south. In the early years of the war Italy was ruled by the fascists led by Mussolini, and Germany and Italy were therefore allies. From the early 1940s there were plans that Germany and Italy would conquer Switzerland together and carve up the country between them. After France's surrender in June 1940, Germany began to prepare for the invasion of Switzerland. Hitler asked to see the plans for the invasion of Switzerland in October of the same year, but they highlighted the highly difficult mountainous terrain. In conquering Western Europe, the Nazis had relied on the Blitzkrieg method of using tanks to break an enemy's line and then encircling them from the air, and then quickly rolling through territory. This had resulted in successful conquests of much of North West Europe (particularly Northern France, Belgium, and the Netherlands, all of which are famously flat). Germany would have needed totally different tactics to invade Switzerland, and the difficulty of adapting successful tactics is seen in the German failure to conquer Britain - they couldn't exactly speedily drive their tanks over the English Channel, and therefore had to engage in a failed air war. After an initial spurt of interest in 1940, Hitler never gave the go-ahead to the plans to invade Switzerland, probably because of the failure to adapt tactics in the war with Britain (and then the USSR), and for the reason of the difficulty of the terrain versus the limited material gain of conquering the Switzerland. Switzerland's neutrality was therefore preserved by a couple of mountains and trees.