Nicholas Burns, a professor at Harvard University, was U.S. undersecretary of state for political affairs from 2005 to 2008. He advised Hillary Clinton’s presidential campaign.

No federal Cabinet agency risks a greater financial hit in President Trump’s first budget than the oldest, the most senior and one of the most vital to our national security: the State Department.

The White House this week signaled that it will seek a massive $20 billion reduction in funding for State and the Agency for International Development, out of a budget of over $50 billion — the highest proportional cuts proposed for any department. If enacted, this proposal would cripple the department’s career foreign and civil service when we need them most.

It would also endanger Trump’s ability to confront the most complex national security agenda in decades. Europe is weaker than at any time since the end of the Cold War, facing Brexit, the refugee crisis, the rise of right-wing populism and an aggressive Vladimir Putin on its borders. The Middle East is in turmoil, with failed states in Libya, Yemen, Iraq and Syria, a still-menacing Islamic State and dangerous Sunni-Shiite tensions dividing the region. China is pushing out in the South and East China Seas. Allies and foes alike are questioning U.S. leadership of the liberal world order. Trump will surely need our diplomats, as well as our warriors, to meet these challenges.

The State Department, however, has had a rough transition from President Barack Obama to Trump. Several of its most senior diplomats have been involuntarily retired by the Trump team. No deputy secretary or undersecretaries of state have been appointed. The seventh floor in Foggy Bottom, where the secretary of state and senior leaders sit, normally pulsates with energy. On a visit this week, it felt like a ghost ship. State needs greater attention, understanding and love from the White House.

State is much smaller than the Pentagon or Homeland Security. It has few large installations and no costly weapons systems that can be delayed or canceled in service to austerity. Its main resource is its personnel. Reductions of the magnitude under consideration would confront Secretary of State Rex Tillerson with an impossible task — cutting deep into the muscle and bone of a foreign and civil service already stretched to the limits. This is simply not a wise path.

The irony is that the State Department is central to what Trump wishes to accomplish overseas. U.S. diplomats interview all would-be immigrants, those applying for U.S. tourist visas and refugees. They assist the thousands of U.S. citizens who find themselves in medical, financial and legal trouble abroad. They deploy as political advisers with our troops in Iraq and Afghanistan and run our highly successful HIV, polio and malaria programs in sub-Saharan Africa. They work directly with U.S. companies to find foreign markets. They negotiate our energy, trade and climate agreements and manage our leadership of the NATO and East Asian Alliance systems so fundamental to the United States’ global power.

As a former Foreign Service officer myself, I admit to a clear bias. But, the men and women of the State Department are a national treasure of language, political and economic experts on places critical to our future — China, Russia, the Islamic world, Latin America and beyond. They comprise the finest diplomatic corps in the world.

Trump’s budget thus illuminates a larger dilemma in the early, chaotic weeks of his presidency. He rarely mentions diplomacy and has given no indication that he values it. If he continues in this vein, it will be a significant barrier to his success. He is right to argue for greater military spending. But he should shift from an exclusive focus on the military and homeland security and join diplomacy to them in pursuit of the stronger America he seeks. President John F. Kennedy recognized this vital link a half-century ago when he said: “Diplomacy and defense are not substitutes for one another. Either alone would fail.”

Trump selected an impressive person in Tillerson, whose life and business experience should translate effectively to diplomacy. The Foreign Service is filled with experienced and capable officers. Given the chance to lead, they will serve Trump with skill, trust and patriotism. But the administration must give them the resources to succeed and plug them into the White House itself. Fortunately, experienced leaders in Congress have already joined the battle on State’s side.

After more than a decade of war, Trump needs to let the State Department rank and file know he believes in them. And he needs to turn to diplomacy to cope with the extraordinary global challenges ahead of him. It could well spell the difference between the success or failure of his presidency.