Nothing was stopping them, other than the practicalities and aims of attacking them. Belgium and the Netherlands were neutral countries too, but obviously were invaded and occupied very early in WW2.
The US was officially neutral until Pearl Harbour, but provided support, Portugal also provided support to the allies and whilst officially neutral, had forces fighting alongside the allies. Spain had forces fighting within the Axis powers too.
Neutrality is just part of the political game a country plays, trying to protect its own interests.
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