To know what "the Nakba" was, it is good to read what the person who coined the term said:
"Seven Arab states declare war on Zionism in Palestine, stop impotent before it, and then turn on their heels. The representatives of the Arabs deliver fiery speeches in the highest international forums, warning what the Arab states and peoples will do if this or that decision be enacted. Declarations fall like bombs from the mouths of officials at the meetings of the Arab League, but when action becomes necessary, the fire is still and quiet, the steel and iron are rusted and twisted, quick to bend and disintegrate. The bombs are hollow and empty. They cause no damage and kill no one.
Seven states seek the abolition of partition and the subduing of Zionism, but they leave the battle having lost a not inconsiderable portion of the soil of Palestine, even of the part "given" to the Arabs in the partition. They are forced to accept a truce in which there is neither advantage nor gain for them...
In short, the Zionist home in Palestine is today closer to realization than ever before. On the other side, the very being of the Arabs has never before known such fragmentation and collapse as in this battle.
In addition to the material collapse is the collapse of values which is shown [in various ways. It is shown] by the doubt of the Arabs in their governments and by their accusations of their leaders and rulers, even by the doubt of many of them in themselves and in their potentiality as a nation. [It is also shown] by the despair in their breasts, by their refusal to meet the danger, and by their shrinking before the magnitude of the blow. Indeed, this moral and spiritual relapse is more important than the material loss, no matter how great the latter might be; for if the resolu- tion of a people crumbles and its confidence in itself is lost, it loses the best that it possesses and is impotent to rise after a fall or to shake from itself the dust of humiliation and defeat.
These are some aspects of the disaster which has overtaken the Arabs in this battle of the Palestine war. These and others, which are on the tips of all tongues, which throb in all hearts, and which every one of us sees and hears [cited] in these critical days, are sufficient to indicate the seriousness of the trial and the intensity of the tragedy...
When the battle broke out, our internal propaganda began to tell us about imaginary victories and to benumb the Arab peoples with the easiness of our military struggle and our ability to overcome and conquer -- until the disaster struck, accompanied by a bitter reaction...
When we view this disaster and appraise its extent and its results, it is also right and fair for us to know that it is only one battle in a long war. If we have lost this battle, that does not mean that we have lost the whole war or that we have been finally routed with no possibility of a later revival...
By advocating that we take far-reaching action and that we view the war in its entirety instead of confining ourselves to the present battle, I do not mean that we should merely await the course of events and depend on favorable circumstances...
I hope that, in the above discussion, I have hit on the truth in describing our present disaster in Palestine and that I have made clear its seriousness and gravity, and the intensity with which it now afflicts us and will afflict us in the future. I also hope that I have pictured the disaster as it really is and that I have outlined the course which we must take in relation to it and the viewpoint from which we must look at it, for this is the necessary first step in analyzing a problem and studying the ways of solving it...
However, whether we are able to strike this crushing blow successfully and establish a united, democratic state in Palestine or whether we prove impotent and have partition imposed on us by the Zionists and the rest of the world, the struggle must go on...
The present struggle, which we have described and whose principles and conditions we have drawn up, is necessary for the battle we are now waging. However, the war waged to uproot Zionism and to conquer it completely will not be finished in a single battle. On the contrary it will require a long and protracted war. To put it briefly -- and in all frankness and sincerity -- this war will not lead to the victory of the Arabs as long as they remain in their present condition. The most that they will be able to accomplish under the circumstances is to guard against the immediate Zionist evil and to protect as much of the Arab being as possible. The road to final and complete victory over this evil is different. The road to this victory lies in a fundamental change in the situation of the Arabs, and in a complete transformation of their modes of thought, action, and life."
--- The Meaning of the Disaster, Constantine K. Zurayk aka Constantin Zureiq (1948), 1956 translation by R. Bayly Winder
There's also quite a lot of racism in the pamphlet, but basically the "Nakba" (disaster) was the failure of the Arabs to eradicate the Jews.
