With President Trump issuing a flurry of executive orders in his first week in office, it’s important for everyone who opposes him to understand the history of this political tool.

Unfortunately for those appalled by Trump’s directives, it cannot be said that the mere issuance of the orders is an outrageous departure from tradition. The truth is that previous presidents have successfully used executive orders to make significant policy changes.

Prior experience also suggests that while it won’t necessarily be impossible to successfully challenge Trump’s executive orders in court — several of George W. Bush’s were — it will be quite difficult, since judges usually interpret presidential power broadly, especially if the legislative branch isn’t objecting.

Kenneth Mayer, a professor at the University of Wisconsin-Madison and one of the foremost academic experts on executive orders, points out that of the thousands of them in U.S. history, “the number of actions that have been explicitly overturned … is pretty small, we’re not talking about tens or dozens or hundreds, it’s really just a handful.”

A subsequent president can rescind them, or Congress can pass laws declaring that it — rather than the president — holds the power on the issue. But that can take an extremely long time.

“The fact that Trump, as all presidents have done before him … is taking action is not by itself unusual,” says Mayer.

Stuart Eizenstat, an attorney who served in high-level posts in the Carter and Clinton administrations, goes further. “While I may disagree with Trump in various areas, I think it’s important in the first 100 days for presidents to show action,” he argues. “Executive orders are a way of getting off to a fast start and showing a sense of direction. … As long as they’re not abused, they’re perfectly permissible and even useful to set the tone of a new administration. That’s what he was elected to do.”

Eizenstat himself witnessed this up close on Jimmy Carter’s first day in office, when Carter fulfilled a campaign promise by pardoning Americans who had evaded the Vietnam-era draft and issued Executive Order 11967 to facilitate the amnesty.

The good news, such as it is, is that the executive branch is only supposed to execute the law, not make it. This means that some of Trump’s executive orders are more symbolic nods to his base than a real change in governance.

For instance, Trump has loudly celebrated his Wednesday executive order calling for “the immediate construction of a physical wall on the southern border.” But presidents can only spend money that’s been appropriated by Congress, for the purposes Congress directed.

So if you read deep into the executive order, you’ll find that it merely instructs the secretary of homeland security to “identify and, to the extent permitted by law, allocate all sources of federal funds for the planning, designing, and constructing of a physical wall.” This means the secretary will look at money already appropriated by Congress and try to find some designated for a purpose that could somehow be interpreted as “build the wall.”

In other words, Trump’s order sets his administration looking under the federal budget’s sofa cushions for spare change, which is not going to add up to anything like the $20-$40 billion the wall would cost.

When Trump excitedly tweeted that “we will build the wall!” it actually just meant “we will build the wall if someday Congress gives me money for it!” — something that was true before he signed the executive order.

Where Trump’s executive orders can have the most impact is in the same places where Obama has been able to act without Congress. He could reverse the Obama executive order which allowed hundreds of thousands of undocumented immigrants brought to the U.S. as children to stay indefinitely. He could also temporarily block visas for anyone from Syria, Iraq, Iran, Libya, Somalia, Sudan, and Yemen. The human damage in both cases would be enormous.

The Emancipation Proclamation was an executive order issued by Abraham Lincoln. An executive order signed by Franklin D. Roosevelt created the legal authority for the internment of Japanese Americans during World War II.

Harry Truman abolished racial discrimination in the military with an executive order. Truman used another to require “loyalty investigations” of every person hired by the executive branch, laying the foundation of political persecution which Joseph McCarthy then enthusiastically built on.

So executive orders are neither inherently good nor bad. They’re an essential mechanism of presidential power that’s been used by every president except William Henry Harrison, who presumably would have issued some if he hadn’t died after 31 days in office.

This is so because it’s impossible for Congress to pass laws that foresee every possible eventuality. Instead, the legislative branch generally sets broad goals and leaves it to the executive branch and its regulatory agencies to figure out how to accomplish them.

This leads to a profusion of executive orders that range from the incredibly picayune, such as ones giving executive branch employees a half day off on Christmas Eve, to the extremely consequential. According to David Pozen, a law professor at Columbia University and former special advisor in the Obama State Department, “the whole classification system” of the U.S. government is essentially a creation of executive orders, with no congressional involvement.

This is a problem since executive orders have been a nebulous concept from the start. As a Congressional Research Service report explains, “executive orders and proclamations are not defined in the Constitution” and “there is also no specific provision in the Constitution authorizing [them]” – hence “the ambiguity behind executive orders and proclamations poses a great concern for Congress and the public.” Until the early 20th century the government didn’t even keep close track of them and many are lost to history.

So it’s tough to confidently judge the legal legitimacy of any individual executive order, including Trump’s. The question of how far a president’s power extends is inherently a political one. If the executive branch overreaches, the whole theory of the U.S. government is that the legislative and judicial branches will — even if they agree politically with the president’s actions — be jealous of their own prerogatives and push back. But theory and practice often differ.

That said, after 241 years as a country we do have codified precedent about how executive orders work. Presidents base their authority to issue them on a combination of Article II of the Constitution (which makes the president the holder of “executive power” and commander in chief of the military, but also requires that he or she “take Care that the Laws be faithfully executed”) and a claim that specific laws passed by Congress delegate the power being wielded to the executive branch.

As professor Mayer says, presidents generally get away with this — but not always. One major exception occurred in 1952, when an incipient strike threatened steel production during the Korean War. In response Truman issued Executive Order 10340, which stated that he was using his power as commander in chief to direct the secretary of commerce to nationalize and operate most of the nation’s steel factories. The steel company owners sued, and the Supreme Court eventually ruled that the president does not have the power to seize private property without authorization by Congress.

Several of George W. Bush’s post-9/11 executive orders were also struck down thanks to lawsuits. Executive Order 13233 clamped down on public access to the records of past presidents until a court ruled that it was partially in violation of the Administrative Procedure Act. Obama later rescinded the order completely. Bush’s Executive Order 13224 declared that 27 organizations were “specially designated global terrorists,” but it was later partially struck down by a judge for being “unconstitutionally vague on its face” because it included no limits on the president’s power to make such a determination.

Obama’s Executive Order 13492, which sought to close the Guantánamo detention facility, was thwarted in a different way, mostly by the legislative branch: Congress passed laws forbidding any appropriated funds from being used to move detainees elsewhere or set up another location where they could go. (Obama himself also bears responsibility for his own executive order’s failure.)

But these are clearly exceptions, not the rule. So where does that leave Trump’s adversaries today?

Eizenstat has been thinking about these issues for a long time, and in an intriguing interview several decades ago objected to liberals focusing on executive orders issued by Ronald Reagan and George H.W. Bush that gave the White House the power to kill or water down environmental regulations. Instead, he said, they should focus on winning elections.

“The law’s always up for grabs,” he argued. “The law is not an inflexible instrument like a cannon that can be lined up and fired. It’s a flexible human instrument that responds to political power. … When you have the power of the presidency, you have the capacity to put people in place who will be sensitive to upholding these laws. When you lose that authority, you’re left with futile rear-guard actions.”

Eizenstat feels the same today. Even worse, he says, is the counterintuitive possibility that with fellow Republicans in control of Congress, Trump may use executive orders more than Obama did as a Democrat with a GOP legislative branch — because Trump will be less fearful of challenges.

Of course, none of this is what anyone horrified by Trump’s policies wants to hear, and any rational person will be alarmed to live in a country where the law is up for grabs. But we do live in a such a country, and the only course of action for Trump’s foes that’s likely to succeed is to get and hold elective political power.