The thesis statement should remain flexible until the paper is actually finished. It ought to be one of the last things that we fuss with in the rewriting process. If we discover new information in the process of writing our paper that ought to be included in the thesis statement, then we'll have to rewrite our thesis statement. On the other hand, if we discover that our paper has done adequate work but the thesis statement appears to include things that we haven't actually addressed, then we need to limit that thesis statement. If the thesis statement is something that we needed prior approval for, changing it might require the permission of the instructor or thesis committee, but it is better to seek such permission than to write a paper that tries to do too much or that claims to do less than it actually accomplishes.
For instance, you might find out that Franco first tried to negotiate with the Axis, but when he couldn't get some concessions that he wanted from them, he turned to the Allies. As you read more about Franco's decisions, you may conclude that Spain's neutrality in WWII occurred for an entirely personal reason: his desire to preserve his own (and Spain's) power. Based on this conclusion, you can then write a trial thesis statement to help you decide what material belongs in your paper.
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