For my essay, I need to establish that patriotism is positively correlated with the level of threat that the country experiences, but I'm not sure

For my essay, I need to establish that patriotism is positively correlated with the level of threat that the country experiences, but I'm not sure how to approach that, or how I should start researching. For example, in 2003 shortly after the 9/11 terrorist attack and the Bush administration's announcement of the Iraqi War, patriotism in the US reached its peak. And it is an anomaly that although the US now faces threats from the ISIS, American patriotism has been declining since 2003. So in order to argue that it is an anomaly, I need to clearly establish that patriotism goes high when the level of threat from abroad goes high. But how should I start researching?  

Patriotism“National identity” is sense of belonging to one’s nation. It creates a feeling of unity andbinds all people belonging to the nation with patriotism. The expression of national...