Argument papers that either support or denounce military action, either by a foreign country or your own, are never short of specific topics on which to write, even during peacetime. Because the military is always moving, adapting to the times, they are always in the news for one reason or another. During wartime, military actions in specific areas can be debated in argument papers.
Social problems and society's reaction to those problems can make interesting argument papers to both write and read. Legal issues such as the passage of a new law either on a federal or local level will have two sides from which to base an argument. Other social reactions might include topics as diverse as reconfiguring the ESRB video game rating system or views for or against same-sex marriage.
Environmental issues can be seen from a political perspective or a personal perspective, making argument papers on environmental topics timely in the sense that your writing could apply to something happening at that very moment. Some environmental topics to consider might be opening state and federal land to oil drilling, wetlands conservation versus need for affordable housing, or even an argument paper that concerns itself with whether or not global warming is actually caused by humanity.
Educational issues are popular topics for college-level argument papers, but have their place in high-school level papers, as well. These issues might include topics such as teacher tenure and union negotiations, public and private school funding, or security on school campus. College level argument papers might concern themselves with tuition funding and scholarships or even a topic as simple as why the cafeteria should not serve tacos on Wednesday.