Assuming you mean Trump’s first administration (2017–2021), the most consequential failures commonly identified by historians, policy analysts, and oversight bodies include:
1. **COVID-19 response** — Testing delays, inconsistent public-health guidance, politicization of masks, and repeated minimization of the threat undermined the response. Operation Warp Speed was a major success, but implementation and public communication were serious weaknesses.
2. **Efforts to overturn the 2020 election** — Trump promoted unsupported fraud claims, pressured officials to change results, and disrupted the peaceful transfer of power, culminating in the January 6 Capitol attack and his second impeachment.
3. **Family-separation policy** — The “zero tolerance” border policy separated thousands of children from their parents without an adequate reunification system, producing lasting humanitarian and legal consequences.
4. **Failure to replace the Affordable Care Act** — Despite repeated promises, the administration never produced or enacted a comprehensive replacement. Its repeal effort failed in Congress, while administrative changes weakened some coverage protections.
5. **Rising deficits and debt before the pandemic** — The 2017 tax cuts did not pay for themselves, and federal deficits increased substantially during a period of economic expansion, leaving less fiscal room when COVID-19 arrived.
6. **Withdrawal from the Iran nuclear agreement** — The “maximum pressure” campaign did not produce a stronger deal; Iran instead expanded its nuclear activities and moved closer to weapons-grade enrichment.
7. **Little durable progress with North Korea** — High-profile summits generated publicity but no binding denuclearization agreement. North Korea continued developing nuclear weapons and missiles.
8. **Trade-war costs** — Tariffs, particularly on China, were largely paid by American importers and consumers, while retaliation harmed farmers and required large federal subsidies. China also fell short of key purchasing commitments under the Phase One deal.
9. **Environmental and climate-policy reversals** — Withdrawal from the Paris Agreement and extensive regulatory rollbacks delayed emissions-reduction efforts and weakened environmental protections.
10. **Poor administrative governance** — Exceptional staff turnover, prolonged vacancies, conflicts of interest, attacks on inspectors general and career officials, and policy made through improvisation reduced institutional competence and continuity.
11. **Failure to enact promised infrastructure legislation** — Despite repeated announcements, no major comprehensive infrastructure bill passed during the first term.
12. **The Afghanistan agreement’s weaknesses** — The 2020 Doha agreement excluded the Afghan government, set a withdrawal timetable, and secured major prisoner releases without obtaining a durable political settlement. The chaotic final withdrawal occurred under Biden, but the Trump agreement substantially shaped the conditions.
The relative importance of these failures is politically contested, and the administration also had achievements—such as Operation Warp Speed, the USMCA, and several Middle East normalization agreements—but those do not erase the shortcomings above.