President Trump made headlines last week by calling for U.N. reform in his first address to the United Nations. Demanding a change to “business as usual” to break with the inefficiency of the past, he encouraged Secretary General António Guterres to “use his authority to cut through the bureaucracy, reform outdated systems and make firm decisions to advance the U.N.’s core mission.”

The suggestion to empower the secretary general directly is one that is likely to be met with both applause and alarm by U.N. reformers. One case from history offers support for both reactions, highlighting both the promise and the challenges for international politics posed by strong leadership at the United Nations.

Dag Hammarskjöld embodied vigorous, decisive leadership during his time as secretary general between 1953 and 1961. Once hailed as a visionary leader who carved out an unprecedented, activist role for the United Nations in world affairs, Hammarskjöld’s tenure ended abruptly with — according to new evidence — his assassination, after his missionary zeal rendered the United Nations morally and financially bankrupt.

Hammarskjöld’s leadership style offers lessons as world leaders debate how to reorganize the United Nations. An activist secretary general can create dynamism at the United Nations, using the power of the organization to do good, but only if the body constructs an innovative procedure to make the secretary general clearly accountable for his actions. Otherwise, empowering the office would simply justify the views of those who think the organization tramples on countries’ interests, thereby weakening itself and harming the cause of global governance.

Hammarskjöld had such success steering negotiations with China for the release of American hostages in 1955 and devising and implementing a peacekeeping mission in Suez in 1956 that through the 1950s the U.S. State Department maintained the slogan: “Leave it to Dag.” As the Cold War dominated global politics, Western diplomats, especially Americans, were content to give Hammarskjöld room to interpret the U.N. Charter in innovative ways. This flexibility allowed him to promote peace by reducing the potential of local conflicts becoming full-blown confrontations between the super powers.

However, the dangers of an interventionist secretary general were soon revealed. When a crisis erupted in Congo in June 1960, Hammarskjöld seized the opportunity to lead the United Nations into uncharted territory. Invoking for the first time his power under Article 99 of the U.N. Charter, he devised a peacekeeping mission that was quickly dispatched to Congo.

By doing so, Hammarskjöld unintentionally internationalized the conflict between Congo and its former colonial power, Belgium. The war, which raged for the next four years, unwittingly drew the Cold War into Africa, challenging European economic interests and forcing Britain and the United States to abandon their previously passive stance at the United Nations.

By September 1961, Britain and the United States publicly broke with Hammarskjöld, criticizing what they called a campaign of U.N. adventurism that challenged their geostrategic interests in Central Africa. The secretary general privately denounced their lack of support, but Britain and the United States insisted that he travel personally to broker a cease-fire in Congo after fighting broke out between U.N. troops and the mercenary army from the mineral rich province of Katanga, which had seceded in 1960.

En route to lead peace negotiations, Hammarskjöld died when his plane crashed in mysterious circumstances, killing all on board. New evidence lends support to decades of rumors that the aircraft was shot down by Western interests unhappy with his handling of the crisis.

These rumors provoked a furious debate about U.N. interference in Congolese politics. His death produced widespread criticism of Britain and the United States, and the expensive peacekeeping mission left the United Nations in a state of moral and financial bankruptcy for decades. Congo fared little better and soon sank into civil war.

The influence and power of the secretary general thus proved to be extremely fluid and led to devastating consequences for the organization as a whole. For the next 30 years, Britain and the United States insisted that secretaries general be moderate and be given only muted support, rendering Hammarskjöld’s successors, U Thant, Kurt Waldheim and Javier Pérez de Cuéllar, largely toothless.

Congo destroyed any notion of the United Nations as an effective, impartial peacekeeper. After U.N. peacekeepers militarily ousted the regime in Katanga in 1962, reasserting the authority of the central government, many states claimed that the United Nations had acted in a partisan manner, violating the most basic principles of its charter. States whose soldiers were killed during the mission also became hesitant to devote troops to future operations, which henceforth were small in size and received only limited mandates. The disaster left the United Nations impotent when it came to conflict resolution for decades to come.

Only in the 1990s under Kofi Annan did a new era of U.N. peacekeeping begin that could be compared to the scale of the Congo mission.

Today, while most stakeholders agree that U.N. reform is long overdue, no current proposal demonstrates an appreciation of this historical context and a clear vision for a reorganized United Nations that would avert the risks revealed by Hammarskjöld’s tenure.

Several schemes for restructuring the Security Council and making the United Nations more effective have recently been presented. One in particular, from a Swedish think tank, proposes precisely what Trump has now also advocated: that the secretary general be empowered to act more authoritatively. Others, such as the Report of the Commission on Global Security, Justice and Governance, present a more wide-ranging vision, directing the United Nations toward more efficient action on issues such as climate change, cybersecurity and fragile states.

Few proposals, however, account for the extent to which the role of the secretary general has depended largely on the character and vision of the individual holding the office. The erratic nature of the job has led to the secretary general being able personally to provide impetus for change or to be a bulwark against it. In addition, he has repeatedly been hamstrung by the failure of member states to support his agenda beyond rhetorical assurances.

While any viable U.N. reform must strengthen the office of the secretary general, this structural reform must be accompanied by the selection of a secretary general who can resolve the tension between national interests and internationalist principles. Hammarskjöld’s example shows that the secretary general can create innovative procedures and foster collaborative communities.

But to achieve this, the secretary general needs to be given room to maneuver without being paralyzed by lack of support from member states pursuing their own narrow agendas. Ceding more power to the secretary general and giving the officeholder sufficient political and material resources to act swiftly and effectively would strengthen global governance overall. A re-energized United Nations would have the capacity to tackle major conflicts, easing the burden on member states to act.

In turn, the secretary general must be more accessible to smaller powers and be more attentive to the role of regional and local organizations. Additionally, by making the office accountable to an advisory committee or a parliamentary assembly with rotating membership, there is no reason that the mistakes of the past should be repeated. If we have an innovative vision for capitalizing upon the potential of the U.N. Charter, someday, we may again be able to “Leave it to Dag.”