When I was in school in Romania I remember my history teacher saying that historically Romania has seen a lot of invaders (referring mainly from the middle ages-present) because it used to have a lot of natural resources. To what extent is this true? I know that Hitler used to get a part of his oil from a city in Romania called Constanta, and right now there is a controversy in Romania because a Canadian firm wants to purchase the rights to a very large gold mine. But historically was Romania invaded for its natural resources? If not, then for what reasons? (I realize that this last question is VERY general, so I don't mind a VERY general answer, such as "usually to expand territory") Thanks.
A full answer would have to do a detailed comparison of resources and relative numbers of invasions across time and territories, which is a bit much, or else track down the motivations of leaders for each particular war, which someone might offer here. Let me get at your question in two ways: 1) What resources were there, and how were they exploited by various states that controlled the territory 2) Why might your teacher have used that particular explanation, and put this into the context of a particular historiography.
For this, let me focus on two sources, cited below in form [Source:Page Number]:
Since it is best to be careful when we transport modern nation-states into the distant past where they did not exist, by "Romania," let us clarify this to refer to the various kingdoms, provinces, and other territories in history which had some form of political control that overlaps with the current borders of that state. These include the Dacian provinces in the Roman Empire, and at various times, Hungary, Transylvania, Wallachia, Moldavia, the Ottoman Empire, Austria, Russia, etc.
The Important Resources
Under Roman control, the resources of the region were "skillfully exploited" [1:7] by the empire. It was an important producer of grain, the gold mines of the Bihor mountains were particularly important, along with lead, copper, silver, iron and salt mines.
Grain, that is, the agricultural wealth of the region continues to be important, but with the many conflicts, the plains of Wallachia and Moldavia were depopulated and did not return to intensive agriculture until after 1829. [1:21] The agricultural sector suffers in general under many conflicts, the power structure that prevents the development of agriculture, and poor harvests in general. [1:82]
Instead, animal husbandry, especially cattle and hide exports, become important in Wallachia and Moldavia in particular and dominate from the 15th to 18th centuries. Oil comes into play in the 15th century in Moldavia and 16th century in Wallachia [1:24] but is not really a major part of the economy until the 20th century (when, as you mentioned, it is a key resource during WWII), though increases significantly after 1859. [1:125]
When Ottoman pressures come to bear on the region, especially on Wallachia and Moldavia, the major extraction from the region is not resources but paid tribute, and later, when they are more fully incorporated into the empire, and during the "Phanariot" period in the 18th century, through direct taxation, make serious and important contributions to the Ottoman war machine. Beyond tributes, the Ottomans extracted other resources, especially grain, cattle, lumber, and saltpeter. [1:77]
Mining doesn't come up that often though, and in 18th century, some efforts to take advantage of copper, mercury and gold mining are abandoned and mining "usually limited to salt" [1:82]
After the establishment of an independent Romania in mid-19th century, however, it grows again to be an important bread basket and by 1913 is the 4th largest wheat exporter in the world [1:125] and in the interwar period up to 1940 becomes the 5th largest agricultural exporter in the world. [1:198]
Overall, the early importance in mining for the Romans, and the importance in grain supplies for both Romans and later rulers were likely a draw, but, as with any territory invaded, the extraction of surpluses in the form of taxation would have been significant, but it would require a much more detailed comparison with other states to determine the relative attractiveness of these territories.
Of course, besides the obvious tribute/taxation and resources, the region is also probably invaded as much as it was, because it happened to be on the way to something more important, especially in the context of the major Habsburg vs. Ottoman and Russia vs. Ottoman conflicts. In fact, Moldavia and Wallachia both actively tried to pursue their own autonomy by portraying themselves, in diplomatic communications to various neighbors, as useful buffer states in 1774, 1783, 1787, 1791, 1807, and 1829. [1:76]
The Historical Narrative
The portrayal of a Romania that is a frequently invaded territory is common to many, but not all nationalist historiographies but the "natural resources" argument may have something to do with preemptively foreclosing other explanations and passing on the idea of stable set of territory which "naturally" belongs to the Romanian nation. Since Romanian nationalism predominantly is based on an ethno-linguistic conception of a language and ethnic group with a common origin (except for some occasional "greater Romania" narratives that pop up over the past 150 years) this means that legitimate claim on territory is predominantly based on a people, language, religion, etc. wherever they are. Perhaps the most interesting recent thing I have seen written on this is:
Holly Case, Between States: The Transylvanian Question and the European Idea During World War II
Which looks at the way the "lost" territory of Transylvania plays between Hungarian and Romanian nationalism in the context of two allies of Germany in WWII. There is a fascinating "battle of the maps," for example, where the predominant map feature is not resources or historical claims, but colorful depictions of demographics. By focusing on claims of where the Romanian people are and how that is to be defined, the claims to territory move beyond historical legitimacy to claims or some other measure, which is true for a lot of 20th century struggles over territorial control, especially around or immediately after WWI. Since Hungary etc. can't justify, your teacher might argue, the argument on the basis on people (though they indeed did, as Case's book shows), they must have just been after the natural resources of the mountainous territories of Transylvania - for example.
Lucian Boia suggests another hint, but it depends on when your teacher was telling you this. Pre-1989? Or after? Boia suggests that around 1993, with publication of The Plot Against Romania (focusing on 1940-1947) and Dan Zamfirescu's War Against the Romanian People there is an increasing theme in historiography to describe "plots" against Romania throughout its history where Romania's neighbors were, Poland-partition style, trying to tear the nation apart and gobble up its various parts. [2:175-178] Perhaps your teacher, if we are talking 1990s here, was influenced by this trend.
I believe Romania's chief misfortune during the Middle Ages was to be right next to Eurasian Steppe, in an era when settled communities really had no military answer to the expert horse archers that steppe country naturally incubated.
Another view (for as long as the link lasts)
To make matters worse, right on the other side of Romania is the Hungarian Plain (or Alföld), which was also perfect territory for those same pastoralists. One can imagine that in the middle in their way was not a fun place to be.
So in a way, yes Romania did contain valuable "resources", if you count good pasture land as a resource where militarily potent pastoralists are concerned.