This is made in the Constructivist style that was big around the 1920s in Soviet Union.
The Combine parallels the 1920s Soviet Union, in that it see's its self as the "nation of the future". 

The Poster promotes Combine air power, meaning their planes can go anywhere in the world.

And this makes the Vex very nervous... 

The phase "Our Wings Over The World" is a nod to the "Wings Over the World", a group of engineers and mechanics that formed a nation that renounced war, promoted science, and outlawed all other independent nations, after a 2nd World War that begin in January of 1940 by Nazi Germany dragged on till the 1950s and resulting with the end of 20th century civilisation.

From the movie, Things to Come (1936)
And the book, The Shape of Things to Come (1933) by HG Wells.