In the game Hearts of Iron 4, in the start date of 1936, the nations of Greece and Yugoslavia have their independence guaranteed by Romania and Czechoslovakia respectevly, what is the historical basis for this decision, if there is any?

Hey, that one's for me! Not only am I interested in that period of history, I am also a somewhat experienced modder (creator of Millennium Dawn) for Hearts of Iron IV, and can provide technical insight to the video game in question.

So, the beginning guarantees in the game are as follows:

Romania guarantees Turkey.
Romania guarantees Greece.
Romania guarantees Czechoslovakia.
Czechoslovakia guarantees Yugoslavia.

Guarantees only work one way, from the guaranteeing nation to the guarantee. And they only trigger when the guarantee is the target of a foreign attack; so an aggressive war will not trigger it. Furthermore, if one guarantee attacks another guaranteed by the same guaranteeing nation, the guaranteeing nation can generally not intervene on either protege's side - that is how the Monroe Doctrine is represented, as the United States starts off guaranteeing all countries in the entire Americas.

Another important thing to note is that you cannot guarantee someone far stronger than yourself, and two nations cannot guarantee each other - guarantees only work one way.

There is currently no way to depict a "defensive pact". The alliance system is game, called "factions", work aggressively as they do defensively, but there is no automatic acceptance into war, so you can refuse to join your ally's war - as Germany and Japan did for a considerable amount of time (even though my historical opinion is that the "Axis Powers" never even existed in the first place, but that there was rather a European Axis and an Asian Axis that existed completely independently of each other, but I am getting off-track).

Furthermore, fascist and communist nations can start "justifying wargoals", as the game calls the action of preparing the immediate act of going to war, right off the bat, whereas democratic and non-aligned nations have to wait for certain thresholds in the "world tension" system.

Alright, that should be the technical side of things out of the way, let's get into the history.

As /u/TheGodDamnedTree already mentioned, Yugoslavia, Romania and Czechoslovakia were historically part of the "Little Entente". The reason why Romania seems to be the driving power in that force in-game (being the one giving two guarantees, whereas Czechoslovakia gives one and Yugoslavia gives none) is its location against both Italy, the primary threat against Yugoslavia, and Germany, the primary threat against Czechoslovakia.

It is however important to note that the Little Entente didn't even really have Germany in mind upon the time of its creation - the primary enemy was Hungary, which after the Treaty of Trianon had to surrender large amounts of territory that either contained Hungarian populations or where the Hungarians pretended they contained Hungarian populations to all three nations. Yugoslavia was from especially 1922 forward also concerned about Italian fascism and Mussolini's influence - Mussolini's irredenta openly aimed at Slovenia and Dalmatia and Italy furthermore controlled the Croatian city of Zadar (Italian: "Zara") on the Dalmatian coast, which at times held as many as 10,000 Italian troops. That is also why Yugoslavia even within the Little Entente always had to fear that turning its back on Italy to aid Romania against Hungary or Bulgaria would just lead to Mussolini, aligned with both Horthy in Hungary and Boris III in Bulgaria, seizing the opportunity and stabbing Yugoslavia in the back.

So that explains these three. Now how do Greece and Turkey come into this? Well, the guarantee of Greece is likely a representation of the Greek–Romanian Non-Aggression and Arbitration Pact, signed as a result of the 1923 Corfu Incident, where Italy bombarded the island of Corfu under the guise of protecting Albania's border interests against the Greeks, and the 1924 cancellation of the Greek-Serbian alliance, where Yugoslavia cut off its diplomatic special relationship with Greece that had existed since 1913. Another of Greece's enemies intent on capturing Greek territory was Bulgaria - Romania felt the same.

As it comes to Turkey? I honestly don't know. I can only guess that this too is about Bulgaria's ambition in eastern Thrace. I honestly can't think of a treaty between Romania and Turkey that qualifies - but as I am generally not concerned with Eastern Europe during this period, the reason is likely my own ignorance.

Sources and Further Reading:

Dirks, Carl: Der Krieg der Generäle. (1999)
Kershaw, Ian: To Hell and Back: Europe, 1914-1949. (2015)
Manstein, Erich v.: Verlorene Siege. (1955)
Piekalkiewicz, Janusz: Der Zweite Weltkrieg. (1985)
Willmott, Hedley et al.: Der Zweite Weltkrieg. (2005)
Wucher, Albert: Seit 5 Uhr 45 wird zurückgeschossen - Ein Dokumentarbericht über Vorgeschichte und Beginn des Zweiten Weltkrieges. (1959)