A single sentence, easily passed over or misunderstood, constitutes the essence of presidential power. It's from Article II, Section 1 saying:
"The executive power shall be vested in a President of the United States of America."
Article II, Section 3 nonchalantly adds:
"The President shall take care that the laws be faithfully exercised," omitting that they can make them through Executive Orders, Presidential Directives and other means, despite no constitutional authority to do so.
Lincoln took full advantage and did what he pleased. He provoked the Fort Sumpter attack and began the Civil War for economic reasons, not to end slavery.
William McKinley created a pretext for war with Spain, annexed Hawaii, colonized Puerto Rico, established a protectorate over Cuba, forced the Spanish government to cede the Phillipines, occupied the country, fought a dirty war, and slaughtered hundreds of thousands of Filipinos. Theodore Roosevelt succeeded him, continued the carnage, and won a Nobel Peace Prize.
In 1916, Woodrow Wilson was reelected on a pledge to "keep us out of war," then in 1917 established the Committee on Public Information that turned a pacifist nation into raging German-haters for the war he planned to enter all along.
FDR waged illegal naval warfare against Germany before Pearl Harbor and, after it, governed as a dictator. Truman atom-bombed Japan twice gratuitously when their leaders were negotiating surrender. He attacked North Korea illegally. So did Johnson and Nixon against Vietnam. Ronald Reagan against Grenada and through proxies in Central America and elsewhere. GHW Bush against Panama and Iraq. Clinton against Yugoslavia and eight years of genocidal sanctions against Iraq. GW Bush against Afghanistan and Iraq, continued under Obama, expanded against Pakistan, and now in occupied Haiti for resources and other exploitive reasons.
In theory, presidents can't violate the law, but can interpret it as freely as they wish. Allied with, representing, chosen and controlled by powerful interests, they can operate largely unconstrained, except when one party seeks political advantage over the other.
Historians call FDR one of the nation's greatest presidents, a widely admired democrat, a leader who freed the world from fascism.
In fact, he was a conservative who partly yielded to necessity after first bailing out Wall Street. Yet he failed to end the Great Depression; did little for blacks, women, immigrants, small farmers, agricultural workers, and the poor; let blacks be persecuted, discriminated against, denied their voting rights and be lynched in the South; interned Japanese, German and Italian Americans during WW II; and gave the public airwaves to private interests.
He tried to save capitalism, not change America into a social democracy, and literally forced the Japanese to attack Pearl Harbor to get into the war 80% of the public opposed.
Zinn wrote this about Andrew Jackson:
"If you look through high school textbooks and elementary textbooks in American history, you will find Jackson the frontiersman, soldier, democrat, man of the people - not Jackson the slaveholder, land speculator, executioner of dissident soldiers, exterminator of Indians."
Others were the same, including George Washington. He envisioned empire, called Native Americans "red savages (and) beasts of prey," dispatched generals to slaughter them, destroy their villages, fields, food supplies, cattle herds, and orchards, seize their land, and take more of it. American imperialism today is global, for much bigger stakes, and nothing deters presidential actions.
From the start, the notion of checks and balances was largely myth. In fact, governments, especially presidents, can and repeatedly have done whatever they wished, with or without popular, congressional, or judicial approval, within or outside the law, and it's no different today.