Cut your timeframe into smaller periods with reasonable justifications. For example, to study the trend in American foreign policy from the end of the Second World War to 2019, it's reasonable and justifiable to use the periods of the presidencies within this timeframe. That is, in reverse chronological order, I would have years of Donald Trump, Barack Obama, George W. Bush, etc.

Hi

Well, You put me interesting question. In my research independent variable presents European army as unified army of EU under single banner and command. Dependent variables are always changing those presents political parties in government, public opinion from think thank academic sphere of defence and European defence study. We can tailored into three essential baskets with different approach to European army project. Conservative parties neglect this project and are at most Natocentric group which acclaim independent national defence policy into alliance, put national forces under government control. Liberal & socialist parties pleads integration policy which includes defence integration of EU member states into EU army, and presents at most Europocentric group. Populist and nationalist pleads dezintegration of European community. So look like proliferation into pubic opinion as dependent variables which are involved into European army project. Do not forget other dependent variables like regional power states as US and Russia those are deeply and actively involved into shaping of European defence policy. US and Natocentric states oppose to build up the EU army under single command.

EU member states policy are always changing into unpredictiblle security in Europe.

Hi,

As for methodological approach, I am dealing with the similar kind of research, so I would like to advise you using either 1) interrupted time series (ITS) analysis (described i.e. at “Experimental and quasi-experimental designs for generalized causal inference” by W. R. Shadish et al., 2002), or, if you prefer qualitative methods, 2) Longitudinal case selection strategy, which is kind of ITS analysis (from “Selecting Cases for Intensive Analysis: A Diversity of Goals and Methods by John Gerring and Lee Cojocaru, 2016).