In all the back and forth over President Trump’s inaugural speech, the press has missed a central point: His address was infused with a wonderful sense of optimism.

As an old Reagan guy, I have learned through the years that optimism equals true leadership. And yes, true leadership cannot be achieved without optimism.

Toward the end of his speech, Mr. Trump said: “We must think big and dream even bigger.” To understand Mr. Trump and his message on Inauguration Day is to appreciate the importance of that sentence.

Mr. Trump then added this: “The time for empty talk is over. Now arrives the hour of action. Do not let anyone tell you it cannot be done. No challenge can match the heart and fight and spirit of America. We will not fail. Our country will thrive and prosper again.”

All the press’s talk about the so-called dark nature of the speech completely obscured these crucial lines.

I don’t know about you, but I am tired of all this talk of American decline. Permanent decline. Secular stagnation. A new normal that dooms us to slow growth. Falling living standards. Weak middle-class wages. And all the rest.

You hear it enough you could almost come to believe it.

Yes, in recent years, the country has fallen into a pessimistic funk. But this is not the America I know. Far more important, it’s not the America President Trump wants.

Mr. Trump was a change candidate, who campaigned against the establishment’s failures at the expense of Main Street Americans. He successfully ran with the simple idea that things can be fixed. He brought that optimism to his inaugural address.

“Now arrives the hour of action. Do not let anyone tell you it cannot be done.”

Decades ago, when Ronald Reagan was elected president, the intellectual consensus was that high inflation, high unemployment, and American decline could not be changed. The idea was that the country was ungovernable.

Reagan put an end to that. He did it with a clear set of easy-to-understand policies to fix the economy and restore American leadership abroad. He guided that plan into place with his quintessential optimism.

Mr. Trump and Reagan are different people. Mr. Trump’s governing style will be nothing like Reagan’s. But the underlying principle of optimism is the same.

“Finally, we must think big and dream even bigger.” How quintessentially American is that? Can we return to being the proverbial City on the Hill? Yes we can.

For these reasons I believe President Trump has the potential to be a transformational figure. He is moving fast. His actions and energy in just the first couple of days have been remarkable.

Everywhere he repeats the theme of economic growth with lower taxes and fewer burdensome regulations. The war on business is over. We will reward, not punish, success.

He talks bilateral trade deals that can be enforced. He is freezing federal hiring, proposing to cut government spending $10.5 trillion over ten years, doing away with Obamacare mandates, getting the Keystone and Dakota pipelines in place, welcoming a constant flow of visitors from businesses and unions, and taking calls from foreign heads of state.

The new president wasted no time in setting up a meeting with Prime Minister May of Britain, moving a U.S.˗Britain free-trade agreement to the front of the queue from the back.

Mr. Trump is making it clear that he will seek border security, replace catch-and-release with catch-and-deport, institute skills-based (rather than family-based) legal immigration, deport criminal illegals, and end sanctuary cities.

Following up on his inaugural pledge to eradicate ISIS — “unite the civilized world against radical Islamic terrorism, which we will eradicate completely from the face of the Earth” — he is calling for a military strategy memo from the Joint Chiefs and is backing an allied coalition of ground forces to take the ISIS stronghold in Raqqa, Syria.

No more containment of ISIS. The eradication of ISIS. We have wanted to hear this for years. Mr. Trump said it, and he means it.

Finally, conservative journalists are now recognized at the beginning of press conferences, cabinet nominees are getting through confirmation, and Republicans on the Hill are finding they can work with the new president.

In all this — from strength at home to strength abroad — Mr. Trump is moving at warp speed. He is keeping to his inaugural pledge that “Every decision on trade, on taxes, on immigration, on foreign affairs will be made to benefit American workers and American families.”

This is what he ran on. Thankfully, he is not about to change. Which is another thing he shares with Reagan. That’s why he has the potential for greatness.

Right now, I truly wish Americans would help him, not seek to harm him. Give him a chance.

“We must think big and dream even bigger.”