Prior to the 1939 German invasion of Poland, Belgium pursued a strict policy of neutrality, known as the Politique d'Indépendance (Independent Policy), formally adopted in 1936 to avoid the devastation of another European war. King Leopold III led this policy by withdrawing from alliances with France and Britain, while shifting toward an independent, strictly defensive military posture, ultimately reinforced by a 1937 neutrality guarantee from Germany. 
 

Key political perspectives in Belgium leading up to 1939:

• Strict Neutrality: Following the heavy damage in World War I, there was almost no appetite to join another European conflict.
• Abandonment of Alliances: In October 1936, King Leopold III declared that Belgium would no longer rely on alliances (such as the 1920 defense pact with France) and would instead focus solely on its own defense.
• German Guarantee: In 1937, Germany, seeking to stabilize its border, formally confirmed its commitment to respect Belgian neutrality.
• Rearmament: While pursuing neutrality, Belgium increased its defense budget and strengthened its border defenses, particularly around Liège, to protect its independence, notes Quora user.
• Internal Political Divides: Pacifist sentiment was strong, particularly in certain political parties, while regional differences (Walloon vs. Flemish) influenced reactions to the changing international situation.

References

•  Trying Not to Lose It - Marine Corps University
•  Belgium in World War II - Wikipedia
• Why did Belgium and the Netherlands remain neutral when ... - Quora
• Belgium Besieged: From Blitzkrieg to Occupation - Warfare History Network
• German invasion of Belgium (1940) - Wikipedia