BLUF: I'm trying to write an essay answering the prompt below and am stuck--I only have one theory.
Prompt: Explain the outbreak of the First World War using theories of international relations and historical information from Margaret MacMillian's The War That Ended Peace.
After reading the book, I've decided to try to explain WWI's outbreak using Walt's realist theory balance of threat. Wikipedia does a good job of summarizing balance of threat. The other theory I would use would have been another realist theory, Jervis' offense/defense theory. However, it seems to me that using two from the same tradition of realism will be redundant. After all, offense/defense theory can be used to support the balance of threat theory.
My professor recommended that I try applying a theory from liberalism or constructivism.
Will you explain how another theory or theories could be applied in explaining WWI? I expect anyone to write my paper for me. It's just that after spending three hours thinking about this, I've realized that I'm out of ideas and am in need for others'.
You will probably want to post this in /r/Ask_Politics, it's not really suited here since it has to do with political theory.
As a side note, realism is easy to apply. Liberalism and constructivism are lesser-discussed theories and harder to apply to world conflict, hence why you're probably having some trouble coming up with a way to fit it into the framework. A basic idea could be that liberalism implies a trust in international institutions, which were lacking and weak prior to WWI. The same could go for the trust in democratic peace theory among liberalist thinkers, which also didn't exist prior to WWI since the nations involved weren't all democracies. Same goes for free trade peace theory.
For constructivism, which is typically a little broader, try looking at different levels of analysis than just the state. Start trying to look at how the identities and ideas of the different leaders involved in WWI might have influenced their propensity to go to war, and their justifications for it. Try looking at how previous wars and collective memories might've influenced the way that each actor made the decisions leading into WWI.
That's the best I can do. WWI is not my speciality, but maybe I've helped point you in the right direction at the very least. If I failed, sorry!