The issue with your question is that your premise is wrong: you’re assuming that the axis wanted to invade and conquer countries, but the whole war was about defeating the UK.
There was no reason to attack Spain for example because Spain was neutral, so it did not pose a threat, nor did it have natural ressources.
Here’s a quick recap of WW2 in Europe: Germany invades Poland to take back land lost in WW1 hoping that the UK and France will not intervene. UK declares war. Because the UK is impossible to invade, Germany adopts a strategy to starve out the UK until they give up (battle of Atlantic begins), and ignores France hoping that France will seek negotiations without conflict (the phoney war, where neither French nor Germans wanted to be the first to shoot despite officially being at war). Poland is defeated, Hitler initiates peace talks but the Allies refuse. Thus, invasion of France begins.
In the North, UK and France try to cut off shipments of iron from Sweden to Germany, and plan to occupy Norway and Denmark. Germany basically undercuts them by invading both Denmark and Norway, preventing the Allies from doing so, and simultaneously protecting their shipping.
Back to France, the French army is powerful and well protected by the Maginot line, fortifications along the border. Germany invades Belgium, Netherlands and Luxembourg in order to bypass the fortifications. With the French army being busy and losing, Italy declares war in order to seize back territories that France had conquered in the past. France gets divided into three, two zones being occupied by Germany and Italy, and the last being autonomous. UK declares war on France to seize their fleet and colonies to help with the war against Germany. With France defeated, Hitler once again asked for peace but the UK refuses. Battle of Britain begins. Fearing what’s happening, the US start preparing for war.
The UK receives a lot of shipping through the Mediterranean. To cut off their supplies and hopefully force them to peace, Italy tries to seize the Suez Canal (but ultimately fail). To protect their oil coming from the Balkans, Germany joins Italy in invading Greece in order to prevent a British invasion.
In Yugoslavia, the British execute a coup d’etat, and Germany immediately invades to prevent the British from attack through there.
In the Middle East, Germany are the ones trying to execute coup d’etat in order to cut off Allie oil supplies. They succeed in Syria, but fail in Iraq.
The Soviet Union, seeing Germany busy on all fronts, and therefore weak, starts building up for an invasion. With the UK refusing all demands for peace, Hitler fears that the Soviet Union has made a deal to join the war on the Allies side (that, plus communism = scary). They initially try to make a pact with the SU, but couldn’t reach an accord, so Germany invades in the hope of undercutting the Soviets, believe that if they wait, the Soviets will be too strong. They were right about that, and ultimately loss and the rest of the war is Germany being defeated everywhere.
As you can see, there was never any random conquests just for the sake of it. The UK was too powerful for a direct invasion, and Germany was too powerful for a direct invasion, and France was too powerful for a direct invasion, therefore the war was fought elsewhere always in the hope of cutting off supplies and forcing the enemy to accept peace. Spain and Switzerland served no purpose and thus were left alone.
The Pacific, on the other hand, was admittedly a whole other mess.
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