Included free with the base game — no Steam store pageFree for a while now, this DLC pulled Hungary, Romania, Czechoslovakia and Yugoslavia out of "pass-through territory" limbo and gave them something real to play.
What New Content It Added
The pack split its additions between four southeastern European trees and one mechanic built especially for small nations.
Nations: Hungary, Romania, Czechoslovakia, and Yugoslavia
Hungary, 
Romania, 
Czechoslovakia and 
Yugoslavia get their first trees of their own, each with routes toward the Axis, the Allies, or more independent paths depending on the country.
Mechanics: Equipment Conversion
The mechanic it introduces is equipment conversion: adapting captured or outdated gear to your current production lines, especially useful for small nations that rely more on battlefield spoils than their own industry.
Nations That Tip the Balance
These four nations decide much of the balance of power in southeastern Europe before Germany or the USSR arrive in force, so playing them well can change the pace of the whole region. Being integrated free, there's no cost to weigh — it's simply a good way to get familiar with minor nations that almost always get ignored.