Without warning, Soviet Union planes came roaring over the city of Helsinki, Finland on November 30, 1939. Finland was about to receive a violent shove into World War II.
The Soviets dropped more than 350 bombs during the raid. Innocent civilians died. Entire buildings were turned to dust. And it was just the beginning. Three hours before the air strike, more than 450,000 Soviet soldiers began marching across the Finnish border.
The Soviet soldiers outnumbered the Finnish army almost 3 to 1. That wasn't the worst of it. The Soviets also commanded more than 6,000 armored tanks and almost 4,000 aircraft. Finland, meanwhile, had just 32 tanks and 114 aircraft. 
It was the beginning of what became known as the Winter War. For
the Finns, there was no question whether some of them would die.
The question was whether any of them would survive.
The Winter War
The winter was brutal that year. In January, temperatures dropped to 40 degrees below zero. Furthermore, at that time of the year and with Finland being located so far north, the soldiers were surrounded by darkness for almost 18 hours per day. Vastly outnumbered, fighting in a brutally cold darkness, and facing near certain death, the Finnish soldiers relied on a concept that has been part of Finnish culture for hundreds of years: Sisu.
Sisu is a word that has no direct translation, but it refers to the idea of continuing to act even in the face of repeated failures and extreme odds. It is a way of living life by displaying perseverance even when you have reached the end of your mental and physical capacities. During the Winter War, the extreme mental toughness of Sisu was all the Finnish soldiers could rely on.
The Finns would suffer more than 70,000 casualties during the Winter War. But that number would pale in comparison to the 323,000 Soviet casualties during that same time. By the end of winter, the Soviets had seen enough. The Moscow Peace Treaty was signed in March 1940. In total, the Soviets had attacked with over 900,000 soldiers during the Winter War. By the end, 300,000 Finns had managed to fight them to a standstill. 
Emilia Lahti, a PhD candidate at Aalto University in Helsinki and former student of Angela Duckworth at University of Pennsylvania, studies the concept of Sisu and how it applies to our lives. According to Lahti, "Sisu is the concept of taking action in the face of significant adversity or challenge. It is not so much about achievement as it is about facing your challenges with valor and determination." She goes on to say, "Sisu provides the final empowering push, when we would otherwise hesitate to act." 
In many ways, Sisu is similar to grit, which has been shown to be one of the best predictors of success in the real world. For example, Angela Duckworth's research on grit has shown that…
- West Point cadets who scored highest on the Grit Test were 60% more likely to succeed than their peers.
- Ivy League undergraduate students who had more grit also had higher GPAs than their peers — even though they had lower SAT scores and weren't as "smart."
- When comparing two people who are the same age but have different levels of education, grit (and not intelligence) more accurately predicts which one will be better educated.
- Competitors in the National Spelling Bee outperform their peers not because of IQ, but because of their grit and commitment to more consistent practice.
(If you'd like more, I wrote about Duckworth's research here.)
But Sisu runs even deeper than grit. It is a type of mental toughness that allows you to bear the burden of your responsibilities, whatever they happen to be, with a will and perseverance that is unbreakable. It is the ability to sustain your action and fight against extreme odds. Sisu extends beyond perseverance. It is what you rely on when you feel like you have nothing left.
Failure is an event, not an identity.
Joshua Waitzkin, a martial arts competitor and champion chess player, says, "At a high level of competition, success often hinges on who determines the field and tone of battle."  It is your mental toughness — your Sisu — that determines the tone of battle.
Most people let their battles define them. They see failure as an indication of who they are. Mentally tough people let their perseverance define them. They see failure as an event. Failure is something that happens to a person, not who a person is. This is attitude is what helped carry the Finnish soldiers through the Winter War. Even when surrounded by failure, by death, and by insurmountable odds, their Sisu did not let them see themselves as failures.
We will all face moments when our physical and mental resources feel tapped out. There will always be times when we are hammered with failure after failure and are called to find a fire within. And perhaps even more frequently, there will be many moments when we want to achieve something, but it feels as if we face incredibly long odds. In those moments, you have to call on your Sisu.
- When you start a business even though you have nobody to look to for guidance. Sisu.
- When you are two miles from finishing your race and it feels as if you can't make it another step. Sisu.
- When you are running on fumes and bleary eyed from caring for your young children, but still need to find the strength to nail your presentation at work. Sisu.
- When you step under the bar and prepare to squat a weight that you have never tried before. Sisu.
- When you're in the middle of a season slump that never seems to end. Sisu.
- When you feel as if you have tried everything you can to achieve your goal and still you haven't made it. Sisu.
We all experience failure, but mentally tough people realize that failure is an event, not their identity. Sisu.
1. Wikipedia entry on the Winter
2. During the Winter War, the Finns employed a series of guerrilla warfare tactics that used the constant darkness and deep snow to their advantage. According to one account , “Small squads of Finnish troops would infiltrate enemy lines between larger divisions and set up machine gun lines pointing outward, towards each division. After short bursts to the left and right, the guerrilla squads would retreat and leave the two, recently alerted adjacent divisions to open fire upon each other thinking they were firing on the enemy when in fact they were firing upon the neighboring division.”
3. Wikipedia entry on Sisu, which includes the work of Emilia Lahti.
4. Conversations on Creativity with Repeat Bloomer Joshua Waitzkin by Scott Barry Kaufman. November 27, 2008.