Now leaving aside controversies that, if they make no comparison to the after-1900 period, should be in another subforum, there is another aspect of the relevant period (1918-1939) to consider: the aversion to alliances.
The alliance systems were widely seen as one of the causes for the cascading declarations of war in WWI. And many thought that the minor powers who got involved were those who fared worse. Even Romania, thanks to siding with the right side, was formidably rewarded in 1918, but had to pay for that with huge destruction and loss of lives.
Belgium had not been in an alliance, it was a neutral - but, it had been covered by a treaty that amounted to a "guarantee". The result was it being turned in one of the main battlefields, with German Schrecklichkeit in the rear areas, to boot.
Conversely, several other minor countries, such as Holland, the Scandinavian states, the usual Switzerland, had remained unscathed and fared relatively well.
So the fad of the 1920s-30s was neutrality. ISTR a Belgian author that argued that without the mutual guarantees over his country, Belgium would have exited the war much less damaged.
The default position for the new states was neutrality. Traditionally neutral states remained neutral. And Belgium (!!!) after having been allied with France, backtracked to neutrality. Invest in an independent army that, albeit small, will be costly to overcome even for a great power, and try to walk a fine line, appeasing here and there.
For a peace-loving, cautious democracy that needs a robust casus belli to risk war, like Britain at this time, such a situation isn't conducive to interventionism. Sending troops to the continent because the Germans are annexing Austria? But Austria never wanted to be an ally of ours. How can we intervene?