WASHINGTON — North Korea's motives for its April 5 rocket launch are open to speculation: a demonstration of its ability to reach out and touch the United States; test-marketing to Iranians who are reported to have observed the launch; a "remember me" welcome to the new Obama administration; or some combination of all of the above. Whatever the motive, it is important to set the rocket test in a broader North Korean political context — the process of political succession that is now under way in Pyongyang.