Leaders are not the only factors that make history, but they are important. Truman, Eisenhower, Kennedy, Nixon, Carter, Reagan and Clinton all instituted major doctrines affecting U.S. foreign policy. However, although leaders’ personalities may be important (Jimmy Carter was brilliant in personally negotiating between Israeli and Palestinian leaders to reach the Camp David agreements), they also operate under many political, legal, historical, organizational and resource constraints.
The most important factor is the extent of the leaders’ knowledge and experience in foreign affairs. Truman had little experience but did fairly well by using common sense and advice from experienced officials. Kennedy had some foreign policy knowledge, which helped in the Cuban Missile Crisis. Johnson had none, which meant that his unthinking anti-communism allowed his advisors to lead him into expanding the Vietnam War. Nixon, a former Senator and Vice-President, had a lot of experience and knowledge; he and Secretary of State Henry Kissinger created the historic opening to China and signed arms limitation agreements with the USSR, but also unnecessarily prolonged the Vietnam War. Reagan and Clinton had little foreign policy experience or knowledge, following their instincts and advisors to mixed records.
Bush 1, a former ambassador to China, CIA chief and Vice-President, had many international connections, which helped him assemble a large alliance in the 1991 Gulf War. It also helped that he followed a policy of restraint when the USSR unexpectedly collapsed. In contrast, Bush 2 had no foreign policy knowledge or experience, which accounted for Vice-President Cheney and other neoconservatives’ leading him into Iraq. Obama also had no foreign policy experience and had to rely heavily on advisors, who convinced him to increase the number of troops in Afghanistan. Trump has no foreign policy experience, but he has definite ideas about how allies exploit the U.S. and how Vladimir Putin of Russia and Kim Jong Un of North Korea are his friends.
Part of attitudes, knowledge and experience is generational. Before WWI, leaders believed that any war between the major powers would be small and short, as most had been for 99 years. Then WWI’s 10 million deaths traumatized everyone so much that they repeatedly appeased Hitler in the 1930s to avoid another bloodbath. When 40 million deaths occurred in WWII anyway, the leaders of that generation took the lesson of avoiding appeasement and stood up to the USSR with containment policy.
Later, the Vietnam War was such a bitter experience that U.S. leaders tried to avoid similar entanglements. Bush 1 grew up when other countries tried to pacify Hitler at Munich, and he was a heroic pilot in WWII. He learned not to appease aggression, and sent troops to push Saddam Hussein out of Kuwait in 1991. Clinton grew up during Vietnam and tried to avoid big military adventures. The 9/11 generation is paranoid about Islamic terrorism, although white nationalists kill far more Americans than Islamists do.
Psychological factors also come into play. Leaders’ beliefs about international politics and their political style can be an important factor. For instance, during the Vietnam War, Lyndon Johnson, a creature of compromise from his years in the Congress, faced hardline nationalist North Vietnamese Marxists who believed in communist revolution and were determined to achieve victory at any cost. He thought his military escalations would convince them to make a deal, but he was wrong.
The Middle East is full of conspiracy theories. Sixty percent of people believe that 9/11 was carried out by the U.S. and Israel. Turkey’s president blames U.S. plots for everything bad that happens. Shark attacks at an Egyptian beach resort were blamed on Israeli intelligence.
Stress can also be an important factor. Stress can result in mistakes, overreaction and lashing out. Japanese leaders in 1941 felt pressured when the U.S. imposed economic embargoes after Japan’s invasion of China, and this contributed to their decision to attack Pearl Harbor. On the other hand, some leaders freeze or have mental breakdowns. After Hitler invaded in 1941, feared USSR dictator Josef Stalin had a nervous breakdown, disappearing for 10 days and spacing out in meetings for several weeks thereafter.
There is also what is called cognitive consistency or confirmation bias. In complex, high-stress situations, people tend to simplify, see what they want to see, and fit the facts to their beliefs and experience. Conflicting information is ignored or dismissed. Before Pearl Harbor, American leaders discounted Japan’s willingness and ability to attack the U.S. Similarly, Japan discounted the U.S. backlash from the attack. In 1967, the Arabs though they could defeat the hugely outnumbered Israelis, but the Israelis carried out a pre-emptive attack that led to a stunning victory. After the success of the 1967 war, the Israelis thought the Arabs would not attack in 1973. When the attack came, Israel suffered heavy losses before they finally prevailed. In Vietnam, the U.S. saw the USSR as the instigator, when the war was primarily a nationalist war against foreign occupiers. Before 9/11, American leaders discounted Al Qaeda’s ability to attack the U.S. Similarly, Al Qaeda discounted American’s willingness to fight back. And going into Iraq in 2003, American leaders believed they would be greeted as liberators and that, despite centuries of autocracy, religious violence and tribalism, Iraqi society would somehow instantly turn into a peaceful, secular, capitalist democracy.
People also use double standards in assessing others. They attribute negative motives and intentions to others but expect others to believe in their own good motives and intentions. Their military buildup is a threat, ours is just for self-defense. Both Arabs and Israelis explain their own actions as necessary due to circumstances and the other side’s actions as proof of bad character and goals. (You may have noticed that this can also happen in personal relationships.)
Another factor is the tendency to make analogies with previous situations. Generals tend to fight the previous war. French generals got ready for another static trench war conflict like WWI and were completely unprepared for the German WWII blitzkrieg, which used airplanes and tanks in high-speed maneuver attacks. The U.S. military prepared for a rerun of the conventional 1991 Iraq war and were not ready for the insurgency in the 2003 Iraq war.
Finally, of course, much depends on the situation. Truman responded to the beginning of the Cold War. Kennedy responded to the Cuban Missile Crisis. Gorbachev ended the Cold War and Bush 1 managed the situation with restraint. Bush 2 rode the wave of American patriotism after 9/11.
So, in general, you can articulate system factors, national factors and individual leadership factors in foreign policy. Hopefully, each situation will be analyzed in all these dimensions, including the relevant specific factors in the immediate situation.