The Belgians very clearly laid out the reasons behind their refusal to allow themselves to become Germany's highway into France in their response to what amounted to a German ultimatum. Germany's request that Belgium allow free passage for its troops through Belgian territory and into Northern France was based on the German claim that it was the French that were planning to invade Belgium in order to strike into Germany across the Meuse river. Claiming this, the Germans made the following declaration:

"The German Government cannot but fear that Belgium, in spite of the utmost goodwill, will be unable, without assistance, to repel so considerable a French invasion with sufficient prospect of success to afford an adequate guarantee against danger to Germany.

It is essential for the self-defence of Germany that she should anticipate any such hostile attack. The German Government would, however, feel the deepest regret if Belgium regarded as an act of hostility against herself the fact that the measures of Germany's opponents force Germany, for her own protection, to enter Belgian territory."

This was clearly a false claim and all sides knew it but Germany was offering Belgium a means to accept German passage without losing face. Belgium of course was not willing to become Germany;s highway and sent the following reply on 3 August 1914:

"This note has made a deep and painful impression upon the Belgian Government. The intentions attributed to France by Germany are in contradiction to the formal declarations made to us on August 1, in the name of the French Government.

Moreover, if, contrary to our expectation, Belgian neutrality should be violated by France, Belgium intends to fulfil her international obligations and the Belgian army would offer the most vigorous resistance to the invader.

The treaties of 1839, confirmed by the treaties of 1870 vouch for the independence and neutrality of Belgium under the guarantee of the Powers, and notably of the Government of His Majesty the King of Prussia.

Belgium has always been faithful to her international obligations, she has carried out her duties in a spirit of loyal impartiality, and she has left nothing undone to maintain and enforce respect for her neutrality.

The attack upon her independence with which the German Government threaten her constitutes a flagrant violation of international law. No strategic interest justifies such a violation of law.

The Belgian Government, if they were to accept the proposals submitted to them, would sacrifice the honour of the nation and betray their duty towards Europe.

Conscious of the part which Belgium has played for more than eighty years in the civilisation of the world, they refuse to believe that the independence of Belgium can only be preserved at the price of the violation of her neutrality.

If this hope is disappointed the Belgian Government are firmly resolved to repel, by all the means in their power, every attack upon their rights."

So in other words, Belgium was willing to defend itself against anyone who violated its neutrality though everyone knew it wouldn't be the French and that the country had a duty to remain neutral and not aid any side in a way that would assist that side in prosecuting a war.

Source: German Request for Free Passage through Belgium, and the Belgian Response, 2-3 August 1914 courtesy of firstworldwar.com