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President Obama at prayer service for Boston Marathon bombing victims (Getty Images)

(The Root) -- Wars and tragedies have a tendency to define a presidency, fairly or not. Lyndon B. Johnson's disastrous handling of the Vietnam War ultimately overshadowed his extraordinary advancement of civil rights. President Franklin Roosevelt's status as one of America's most beloved presidents was solidified with his leadership during World War II. President George W. Bush's response to the Sept. 11 attacks, including launching unpopular dual wars, made him one of the least popular presidents on record when he left office.

With this week's Boston terror attacks, Barack Obama now faces one of the greatest tests of his presidency. How he responds in the weeks and months to come will largely shape his long-term legacy. His presidency has already seen more tragedy than many others. Five of the worst gun massacres in U.S. history have taken place since President Obama took office.

But this week is different.

While the Newtown, Conn., massacre, in which schoolchildren and staff were murdered by a deranged gunman, united Americans in tragedy, it immediately divided us politically over the issue of gun control. That divide has become particularly pronounced in recent days, as the Senate voted down a measure to expand background checks (which nearly 90 percent of Americans support) and as a member of Congress faced death threats for her gun control advocacy.

But the Boston attacks have, for now, united us in more universal ways: in support of the victims and the city of Boston as a whole, and in support of being Americans -- regardless of our political parties and any of the other labels we usually use to divide one another.

Now that we are one for this brief moment, the question becomes how the president will leverage this unity in his leadership. President Bush leveraged it into two wars -- wars that even members of his own party denounced. With the investigation into the Boston culprits and their motives ongoing, it is far too early to predict what type of policy fallout there may be from this tragedy. But it is not too early to predict that whatever that fallout may be, it will have a lasting impact on America, much as 9/11 has.

But it will also largely define the legacy of the first black president.

After a first term spent being weighed down by the pettiness that has come to define Washington, including attacks on his character, his race and even his family, President Obama got a powerful reminder of what is really important this week. Here's hoping that he and his supporters, as well as his critics, don't forget that reminder anytime soon.

Here's hoping that President Obama, who has spent a week serving as our comforter in chief, effortlessly rises to the occasion and gives America the commander in chief it needs during this dark time and in the days ahead. We, as Americans, need him to. His speech in honor of the victims, in which he said with conviction, "We will finish the race," gives me hope that he will.

After a week filled with such terror and tumult, hope is all we can really hold on to. 

Keli Goff is The Root’s special correspondent. Follow her on Twitter.

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