The State Department and foreign aid have long been targets for budget cutters because many Americans don’t understand what these programs do. Polls show that Americans overestimate how much federal spending goes to these programs. The actual number for foreign aid is about 1 percent of the budget, or $36.6 billion in 2017. And some of that money is spent in the United States.

Diplomacy doesn’t always prevent war, Syria being one example, but war becomes far more likely if there are not enough diplomats to work with other countries to resolve disagreements. Compelling examples of diplomacy working include the 2015 deal that is preventing Iran from developing a nuclear weapon; the 1995 Dayton Accords, which ended the Bosnia War; and the 1979 Israel-Egypt peace treaty. Other examples include several treaties that committed America and Russia to reduce their nuclear arsenals significantly. American diplomats have strengthened alliances, built new partnerships with countries like Cuba and Myanmar, promoted democracy so that countries are less likely to go to war with one another and created jobs by helping to open overseas markets to American business.

American interests are also advanced by helping other countries become more stable. A health program begun by President George W. Bush helped check the spread of AIDS in sub-Saharan Africa and then was used to stop an Ebola epidemic in West Africa during the Obama administration. Other programs aid refugees; finance improvements in sanitation and water, primary education, energy and counterterrorism; and underwrite exchanges between foreign students and professionals and their American counterparts.

None of this is to say the State Department cannot be made more efficient. Tax dollars should be spent wisely. But rather than slashing the department’s budget, which pales before a $600 billion Pentagon budget (which alone exceeds the military spending of the next seven countries combined), Mr. Trump should be urging Congress to increase it.

Secretary of State Rex Tillerson, a former Exxon Mobil chief executive, has been shockingly complicit in Mr. Trump’s miserly approach. In Tokyo this month, he called the department’s current spending “not sustainable” and said that “as time goes by, there will be fewer military conflicts that the U.S. will be directly engaged in.” If that statement were true, it would be an argument against increasing Pentagon spending, not for cutting money for diplomacy.