Before answering, we must note that there is a major difference between something that is “lasting” and something that is “successful.” I would argue that it is easy to see what Theodore Roosevelt’s most lasting accomplishment is, but the issue of whether it was successful is more of a matter of opinion.
I would say that Roosevelt’s most lasting accomplishment was that he changed the American system of government. Before Roosevelt, the government was relatively small and did not try to do very much. In addition, the presidency was a rather weak institution that was not central to the American political system. Roosevelt changed these things in ways that have been very lasting.
In our time, the president is by far the biggest player in the American political system. Everything revolves around the president. We look to him (or someday her) to provide an agenda for where the country is going. This was not the case in the days before Roosevelt was president. When Roosevelt came to office, the force of his personality helped to make him the center of American politics. This focus on the president lasted past his time in office and continues to this day.
In our time, the government is involved in many aspects of our lives. This is largely a legacy of Roosevelt and the other progressive presidents. The progressives wanted the government to be more involved in creating what they saw as a more ideal society. It was for this reason that Roosevelt pushed the government to do things like setting up national parks and regulating big business. Since Roosevelt’s time, the level of government involvement in our lives has grown tremendously. This can be seen as another aspect of his legacy.
Roosevelt, then, accomplished two major changes in our system of government. These changes have been very lasting. Some would say that they have been good (successful) changes, but others would argue that they have been bad for our country.