Can you identify a foreign policy action of one developed nation that drastically affected a developing nation?

One state, or one country? In the USA we don't normally allow states to make foreign policy decisions, and haven't since the civil war, when the southern states seceded and used Bermuda as a base for blockade runners to smuggle cotton out.

Great Britain had declared neutrality, officially, in spite of the efforts of the French to persuade them to recognize the confederacy. The economy in Great Britain at that time was heavily dependent on southern cotton.

Supplies from France traveled up through Mexico, with the knowledge of the Mexicans, but the Confederacy did not work through the official Mexican government. Instead, they allied with warlords in northern Mexico to funnel the French supplies.

Russia openly and verbally declared their willingness to assist the Union, as a means of trying to obtain an ally against Great Britain.

Well if you look at the War in Iraq, the foreign policy of the USA led to war.

-what period are we talking about?
-directly affect or a country that was affected by two different countries foreign policy?
-affect in what sense? Politically? Economically?

I believe that Germany's foreign policy in invading Poland in 1939 affected quite a few others as it led to the Second World War.

England's war with France in 1513 got Scotland involved

The Balkan squabbles of 1914 created the First World War as well

The Russian Japanese conflict over Manchuria set Japan up for WW II and was the 1st nail in the coffin of the Russian empire. This conflict had massive influence on the 20th century.

Bombing other countries counts?