Marcus Licinius Crassus was 62 years old. He was the richest man in Rome, decorated with honors, and a member of the secret ruling Triumvirate with Julius Caesar and Pompey Magnus. But it wasn't enough for him.
In renewing his political alliance with Caesar and Pompey, they dealt out assignments and power to the members of the Triumvirate. Crassus would go southeast to fight and conquer the Parthian Empire.
Crassus was the classical expansionist. But he made the fatal error; he did not take his next opponent seriously.
As he marched his legions to fight the Parthians, he was bragging that after Parthia he would keep going -- maybe all the way to India.
He was ill-prepared for the Parthian mounted horse archers, some of the most dangerous in the ancient world. Even then, Roman allied and Crassus's officers alike recommended plans that would have saved Crassus's force if he hadn't blundered forwards.
His eyes were off the next engagement. He was looking two wars hence. Already the wealthiest man in Rome, he wanted more. And he tried to get there, not by focusing on the current moment and immediate next steps, but by daydreaming about future greatness.
The Parthians captured him, and poured molted gold down his throat to execute him in a symbolic gesture.
Expansionists burn bright and rapidly change the world, but are most often annihilated and lose all the gains they made.
If you've got the expansionist's streak in you, absolutely be careful to take the next opponent seriously. It is the classical failure mode for expansionists. Overexpansion often gets beaten by a weak opponent that was entirely preventable.
Napoleon's tribulations in Spain tied down his top troops and destroyed his moral authority him long before he marched into Russia -- and why did he attempt to topple a semi-friendly ally while still at war with the great threat, England? And then, he didn't take the campaign seriously, and guerrilla war broke out, and... well, Napoleon died in exile on a tiny island, most of his gains rolled back.
Now, "expansionist" doesn't mean "ambitious" by itself, nor does "consolidationist" mean "unambitious."
Sometimes, maybe. But Tokugawa Ieyasu was certainly ambitious, yet his advancement was surefooted and steady. There were only perhaps one or two moments in his life where his life was in immediate danger; by avoiding those moments where things could break either way, he didn't leave himself many opportunities to fail. And he always focused on the matter at hand while balancing the future, before going on to unify Japan under the Tokugawa Shogunate.
I would argue that one of the reasons of the United States's greatness is that we've had an immense amount of ambitious leaders, and yet we've gotten consistent consolidation in history. The Bill of Rights to the Constitution itself is a classical consolidation measure, putting safeguards and protections down permanently instead of trusting the future while rushing forwards blindly.
Even while expanding towards control of the entire American continent, American leaders went rather slowly and took it, generally, one thing at a time. When a single event showed vulnerability -- the War of 1812 showing that American ports were completely vulnerable to a powerful fleet -- American leaders adjusted and built.
The most expansionist American leader was Franklin Delano Roosevelt, and his governance was immediately followed by consolidation under Harry Truman and Dwight Eisenhower. In these cases, occasionally the President and Congress would underestimate a looming threat -- they didn't realize what a monster the Soviet Union would be until it was too late to roll back their Eastern European conquests -- but they almost never neglected the moment at hand and thought multiple wars and campaigns out.
This is a virtue. Look past the present, sure. But briefly. Don't daydream. Take the next challenge seriously. Prepare completely and follow through until completion and certainty of success; do not make Crassus's error in looking past the current challenges into daydreams of grandeur.