You are going to have to delve into this one. I think that the challenge is going to be finding an area in U.S. affairs abroad that holds interest and fascination for you. This could be the war on terrorism, how the United States is doing in this endeavor, the changing role of the United States in world affairs, or perhaps, how the US economic crisis is impacting other nations. Another one might be how the United States has to work through the challenges of outsourcing jobs to other nations or its debt being bankrolled by other nations. For example, with a huge deficit and loans to sustain us, do we as a nation really want to own nations like China trillions of dollars? What if they call in their chit? What do we do? I think being able to find something along these lines might be a good start. The next step would be to formulate your opinion on that topic. This is where the web is a great resource in that you can access information on the United States' foreign presence quite easily. Being able to determine where you stand on the particular issues is going to be vitally important. Within this, being able to identify a challenge and then your approach or ideas on it, you will have an editorial for your assignment.
A newspaper's job is to objectively report the news, however, the editorial page is the only place where an opinion is supposed to be okay (as you know, many journalists do not follow this rule and examples of bias can always be found in supposed news articles...but I digress).
For your editorial, I suggest you choose a specific event in which the United States is curently involved in the affairs of other countries and then express your opinon about it. You should research your topic so that you can give supporting evidence because even though an editorial is an opinion, it must be supported by facts.
Some suggestions? The war in Iraq or Afghanistan; U.S. involvement in the crisis in Sudan; the U.S. involvement in the current situation between North and South Korea (North Korea supposedly sunk a South Korean submarine and the U.S. Navy is patrolling the area right now...I know this because my son is a naval aviator on that ship!). Or, how about the U.S. involvement in the crisis in the Middle East? Or, what about the U.S. involvement in the Mexican drug wars? This international event affects the United States, so we have good reason to care about what happens there. Right now, it is unsafe to travel to Mexico. There is a lot of information available about this.
If you do an online search of "U.S. international relations" you will find some other topics that might be of interest to you.