Thursday: President Obama and Russian President Dmitri Medvedev will meet in Prague to sign a nuclear arms control agreement that will reduce their nations' arsenals by 30 percent.

Next Tuesday: Obama will host 47 world leaders in Washington for a summit on nuclear security. Obama hopes to use the session to lay down tangible commitments by individual countries toward his goal of securing the world's nuclear material so it does not fall into the wrong hands.

In May: Nearly all governments in the world will meet in New York to review the operation of the Nuclear Non-Proliferation Treaty. The last review in 2005 ended in acrimony and some predicted the end of the treaty.