It is all too easy to look back on some moments in the past and claim their significance was obvious at the time. That it was clear – even as the event unfolded and you frantically scribbled details down in a notebook – that you were witnessing a historic moment.

Certainly that was the case when Barack Obama gave the keynote address at the 2004 Boston convention that anointed John Kerry as the champion of a Democratic party frantic to defeat George W Bush. Obama – then a state senator from Illinois – delivered one of the finest speeches in modern times. His oratory lit up that dull Boston shindig like 4 July fireworks.

Or at least I remember it that way. The morning after his speech, I trailed Obama through a series of events, seeking an interview. It was too late. He was already becoming a political rock star, viewable only behind a freshly minted posse of minders.

Change happens quickly in America; it is part of the genius of the nation. Four years after that night, Obama was in the White House, having ridden a wave of hope and optimism that made him a global hero and America's first-ever black president. Four years on again, he would win a second term, defeating Mitt Romney and the Tea Party hordes at his back.

Surely that early promise of Boston had been fulfilled. Everything presaged in that remarkable 2004 speech had come true. I had indeed witnessed history in a glorious moment of its making. Sadly, I now think not.

Over the last two weeks, the world has seen an extraordinary series of revelations about the scale, size and activities of the National Security Agency under Obama's administration. Though he came to power decrying the secret actions of Bush, Obama has embraced and extended many of the same activities. His NSA uses a secret court system to get permission for its shadowy work, hauls out "metadata" on millions of Americans' phone calls, taps into the biggest and most powerful internet companies of the information age – Facebook, Skype, YouTube, Yahoo, Google – to monitor and snoop. Its tools have names like Prism and Boundless Informant, as if their inventors were all too aware that they resembled dystopian science fiction.

Yet Obama has flippantly dismissed the controversy. Resorting to the worst tactics of the Bush years, his message is: "Trust us. We're the good guys." And then Congress is briefed – in secret, of course – about the "dozens" of terrorist plots such industrial-scale espionage has stopped.

No one in that hall in Boston in 2004 could have imagined that the young, eloquent and inspiring politician would have transformed so dramatically less than a decade later. Yet the age of Obama is not one of hope and change; it is the era of the national security president. Obama has overseen increasing use of drones, in a targeted killing programme across the globe. No doubt they wipe out legitimate targets. But the drones also murder American citizens, such as Anwar al-Awlaki and his son Abdulraman in 2011, with no trial, amid a legal framework that – again – is kept largely secret. They wipe out wedding parties by accident. Any "military-aged male" in a drone strike zone is called a legitimate target, turning the innocent into the guilty to justify death from above. Then there is Guantánamo Bay, that bleeding sore on the face of American civil liberties. It is a tropical gulag of 166 men – more than half cleared for release but still kept behind bars – who are starving themselves out of desperation. Obama promised to close it down in 2008. He failed. He promised again last month. But nothing has happened. Meanwhile, the regime inside the camp is growing more savage.

Obama has cracked down aggressively on whistleblowers, using the Espionage Act – a hangover from the first world war – more times than all his predecessors combined. He has presided over an explosion of over-classification, as millions of government documents are shuttered away from public eyes. His Department of Justice has collected the phone records of AP journalists and accessed the emails of a Fox News reporter.

It's the stuff of conspiracy theorist fantasies. But these abuses of power are real and are playing out on the front pages of America's papers every day. When the IRS searched for conservative groups to target for special treatment, it confirmed the worst fears of every rightwinger in America.

How on earth did we get here from Boston 2004? Bush – a cipher of a politician whose only belief was in his right to rule – surrounded himself with Dick Cheney, Donald Rumsfeld, John Bolton and an army of whispering neocons. Obama does not have that excuse. When his staff meets to mull over the latest names in their killing programme – an event dubbed "Terror Tuesdays" – Obama himself is often present.

Neither is Obama ignorant of the law; he's a constitutional law professor. In turning America into a national security state, the awful truth is that he knows full well what he is doing.

There are three more years of this to come. Involvement in Syria's war looms, and more terrorist attacks like the one that hit the Boston marathon could lurk in the future. Where will Obama take America in that time? Judging him on his past actions, I think it will be no place good.

Due to his race, Obama is often cast in the light of America's civil rights movement and its heroic leader, Martin Luther King. Among King's most famous words are his hopes that his "four little children will one day live in a nation where they will not be judged by the colour of their skin, but by the content of their character".

That dream of King's was what many believed Obama would one day fulfil. Perhaps he has, just not in the way anyone thought. In 2013 – amid drones, assassinations, mass spying, secret courts and tapping journalists' phones – it seems that Obama's race matters less and less, while his inner character is shining through for judgment. It is sorely wanting.