There are scores of ways to evaluate the beginning of a presidency. One of the most facile is to look at a president's first 100 days, which in the case of President Barack Obama ends Wednesday, and compare the achievements (or failures) with the first hundred days of Franklin Delano Roosevelt. With an economic crisis raging -- though not one as severe as 1933 -- that temptation is going to be irresistible.

Let's let others do that, remembering of course that the phrase "hundred days" comes not from FDR but from Napoleon, and from a far less noble moment in history -- the French emperor's return from his Elba exile and the threat he posed to order in Europe.

For our purposes, let's let the Obama presidency create its own rhythm, but let's use Harry Truman, rather than Roosevelt, as our guide. And let's draw our perspective from the very first page of Volume II of Truman's memoir, with its beguilingly appropriate title, "Years of Trial and Hope" (Doubleday, 1956).

Within the first few months I discovered that being a president is like riding a tiger. A man has to keep on riding or be swallowed.

Harry S Truman and Obama came to office in vastly different ways, the one after the death of a popular president who so dominated his era that many people could not remember a time when Franklin Roosevelt did not occupy the White House, and the other after the expiration of the term of an unpopular president whose policies laid the groundwork for his insurgency. But both men inherited wars in two parts of the world that were unfinished and unwon.

The years 1945 and 2009 both were years of great passion and peril, years when the world as Americans knew it seemed to hang in a precarious balance. These two presidents had much to do, little time to do it, and they had to perform the duties of their office even as they had to win the allegiance (in the Truman case) or the confidence (in the Obama example) of a worried public.

A president either is constantly on top of events or, if he hesitates, events will soon be on top of him. I never felt that I could let up for a single moment. 

Both Truman, with wars in Europe and Asia, and Obama, with wars in Iraq and Afghanistan and an economic crisis at home, faced crowded hours, the sense of urgency and emergency bearing down on them remorselessly. Both showed considerable grace under that pressure, in the face of opposition that grew in visibility and decibel level (but, in the Obama case, not yet in the polls).

No one who has not had the responsibility can really understand what it is like to be president, not even his closest aides or members of his immediate family. There is no end to the chain of responsibility that binds him, and he is never allowed to forget that he is president.

The pressure is relentless, unforgiving. The subjects that cross his desk are numberless, unpredictable. Truman faced the imminent collapse in Europe of perhaps the greatest tyranny that ever existed and the sobering challenge of defeating an expansionist tyranny in Asia that stubbornly remained at war. Obama is facing terrorist threats, economic turmoil and nuclear proliferation among rogue nations and, worse yet, rogue groups. The perspective of the Truman years helps us see that what was at stake for the 33rd president is the same thing that is at stake for the 44th: a re-ordering of the world's power centers even as the world economy is being re-created.

By nature not given to making snap judgments or easy decisions, I required all available facts and information before coming to a decision. But once a decision was made, I did not worry about it afterwards.

Truman was known for his aversion to second-guessing or backing down, Team Obama less so. The Obama White House has pushed through a massive stimulus bill and budget, but also has proposed several ideas -- trimming the home mortgage deduction for the wealthiest of taxpayers, adjusting the amount of charitable giving that was tax-deductible, opposing earmarks, substantially cutting farm subsidies -- only to back away from them. The result is a Washington mystery: What is the president willing to spend his political capital on?

I had trained myself to look back into history for precedents, because instinctively I sought perspective in the span of history for the decisions I had to make. That is why I read and re-read history. Most of the problems a president has to face have their roots in the past.

Like Truman, Obama is a student of history. But students of history know that the answers to the problems of the present are seldom found in the past; history helps explain the world we inhabit, but it does not provide a path to the future. This is a lesson Obama obviously has heeded; his policies toward Cuba, for example, represent a sharp break with the past. They are the product of a man who understands why Washington and Havana have been in a dance of death for a half-century -- and who understands that such a standoff is unsustainable as we approach the second decade of the 21st century.

Now a quick look at the last paragraph of the Truman memoirs, a reference to a session with his successor, Dwight D. Eisenhower:

I had the feeling that, up to this meeting in the White House, Gen. Eisenhower had not grasped the immense job ahead of him. There was something about his attitude during the meeting that I did not understand. It may have been that this meeting made him realize for the first time what the presidency and the responsibilities of the president were. He may have been awestruck by the long array of problems and decisions the president has to face. If that is so, then I can almost understand his frozen grimness throughout the meeting.

Obama projects an ease with power and an ease with his burden. His power may dissipate even as his burden increases. But he asked for the job. In the hundreds of days that follow the first 100, the thanklessness only grows.