Remarks in italics are not taken from explicitly-stated events in the canon material. They are my own speculations, logical inferences, gap-fillers, and extrapolations based on fragmentary references and passing mentions in the original sources.
Early 1915 — Professor Henry Jones, Sr. returns to teaching at Princeton full-time, and Indiana completes his sophomore year of high school.
Indy, who as a youth never met an odd job he didn’t like, hires himself out as an errand boy for busy Princeton students. Indy probably loathed asking his father for spending money, and liked to be out of the house as much as possible.
Summer 1915 — There is likely a round of summer school for Indy after missing his entire fall semester last year, and hopefully a trip to the New Mexico ranch he has grown attached to. His Uncle Fred and Aunt Grace provide the emotional stability his father does not, his similarly-aged cousin Fred, Jr. is one of his best friends, and ranch work agrees with him.
At some point this summer, Indy works shoveling coal at a train depot, either in Princeton or New Mexico. 
Autumn 1915 — Indy begins his junior year of high school. He gets a job as a soda jerk at the drugstore downtown. He also begins dating Nancy, the daughter of famous author Edward Stratemeyer, creator of the Hardy Boys and Nancy Drew. Stratemeyer bases a lot of Nancy Drew on his daughter, who is independent and intelligent and loves mysteries.
February 1916 — Indy begins spring training for his high school baseball team.[3,4]
Late February, 1916 — Indy is desperate to borrow Mr. Stratemeyer’s electric Bugatti car to take Nancy to the prom, but it needs a new generator. Indy takes the generator to an acquaintance of his father’s at Edison Laboratories in West Orange, N.J. Important plans for a long-lasting, powerful battery are stolen (along with, accidentally, the Bugatti generator). At first, German spies are suspected, but Indy and Nancy discover that a big oil company is behind the theft.
February 26, 1916 — Indy and Nancy attend the prom in triumph, much to the consternation of Indy’s rival, Butch.
March 5, 1916 — Indy arrives at the New Mexico ranch for his spring break, accompanied by his father.
March 8, 1916 — Indy and Fred, Jr. leave for a camping trip, promising to be back in time for Indy’s departure for Princeton on Sunday the 12th. What they actually intend to do is hitch a ride to the border town of Columbus, N.M., and visit the brothels. They arrive to find Columbus seemingly abandoned, but really just anticipating a violent raid by Pancho Villa and his Mexican revolutionaries. Indy and Fred get caught in the crossfire when the raid actually happens. Fred escapes, but Indy is taken by Villa’s men.
Mid – Late March, 1916 — Inspired by Villa’s dedication for fighting for “the people,” Indy quickly goes from hostage to participant. He meets Remy Baudouin, a hedonistic and temperamental Belgian ship’s cook several years his senior, who had jumped ship and settled in Mexico. Remy joined Villa after the federales raided his cantina and killed his wife. Indy also notices that one of the people Villa is trading with for weapons looks vaguely familiar and speaks Arabic. After spending a few days with Villa and his men, and observing their tactics which include theft and looting, Indy decides the revolution is hurting the people as much as helping. Upon seeing newsreel footage of the ongoing Great War, Remy decides to return to his homeland and fight for Belgium. Indy agrees to join him, but before he can do that, he finally recognizes Villa’s arms dealer as Dimitrios, the man who stole the jewel from the Egyptian tomb back in 1908. Indy recovers the jewel and rides to meet up with Remy in Veracruz and find a ship going to Europe.
Early April 1916 — On the ship steaming from Mexico to Ireland, Indy and Remy have hidden themselves as stowaways. They are found out and put to work in the engine room. Indy gets them back in the captain’s good graces by overhearing a German saboteur attempting to induce the Mexican crew to mutiny. He tips off the captain, but not before nearly being tossed overboard.
Indiana the dog dies in Princeton of old age as Indy is on his way to Ireland.
April 1916 — Upon arrival in Dublin, Indy and Remy go to work as barmen in the local pub to afford the remaining fare to London, where they intend to enlist with the Belgian army. Indy’s flirtation with a young Irish girl, Maggie Leamass, prolongs their stay as he keeps spending his money on outings with her rather than saving it. Her brother Sean, who gradually builds a friendship with Indy, is a militant Irish Republican.
April 24, 1916 — Indy witnesses the Easter Rebellion, a failed attempt to end Britain’s rule of Ireland and establish a free Irish republic. Many of the people known to Indy from his work at the pub and friendship with Sean and Maggie Leamass are involved.
May 13, 1916 — Indy and Remy leave for London as soon as the final executions of the Irish Rebellion ringleaders are complete. (They had stayed to offer the Leamasses moral support.)
Mid-May, 1916 — In London, Indy and Remy officially enlist in the Belgian Army. Indy gives his age as 22, and his name as “Henri Defense.” (Remy indicates they would have taken him no matter what.) While waiting to be called up for service, Indy begins a relationship with a well-educated, outspoken English suffragette, Vicky Prentiss.
Late May, 1916 — Indy thinks enough of Vicky to invite her to Oxford to visit his old tutor, Helen Seymour. Miss Seymour does not approve of Indy’s recent life choices, and insists Indy write his father (who still thinks he is in Mexico) and update him. Miss Seymour invites Indy and Vicky to a dinner party with Winston Churchill, who has words with Vicky on the topic of votes for women. Back in London, Indy proposes marriage to Vicky, who declines. Indy and Remy’s call-up papers arrive, and they set off for the battlefield.
June 1916 — “Henri Defense” and Remy are now privates in the 9th Belgian Infantry, and participate in basic training at Le Havre, France. During his downtime, Indy acquires an old soprano saxophone and teaches himself how to to play it.
July – August 1916 — The 9th Belgian Infantry goes into combat in Flanders. Indy and Remy get their first taste of vicious trench warfare, seeing heavy action, witnessing the death of several comrades, and losing all of their company officers. Indy, at some point having been promoted to corporal, is in charge of the decimated unit when it is finally pulled from the front lines.
August 1916 — The much-reduced Belgian 9th are attached to the 14th French Infantry, and thrown into the Battle of the Somme, one of the bloodiest battles in the history of warfare. Their first assault on the high ground ultimately fails in the face of heavy German machine guns, poison gas, and flamethrowers. The second assault (after a two-day break), initially succeeds, but they don’t have enough troops to hold the position, and the Germans counterattack. Remy is severely wounded, and Indy is taken prisoner. At the prison camp, he participates in a tunnel escape, which fails. Because he is caught wearing a coat with the ID of another soldier (a repeat escapee), Indy is sent to Dusterstadt, a fortress on an island in the Rhine River being used as a maximum security prison for “incorrigibles.” There he meets Charles De Gaulle, another incorrigible repeat escapee. They devise an escape plan (leaving the prison inside the coffins of two dead prisoners). DeGaulle is re-captured, but Indy makes it to freedom.
September 1916 — Indy has returned to the Belgian Army, and is now working as a motorcycle courier, taking messages from the trenches to the high command in their palatial estates far from the front lines near Verdun. He witnesses yet another failed frontal assault across no man’s land. The French generals are growing increasingly desperate. Indy, who speaks fluent German, volunteers for a dangerous mission — sneaking over to the German lines under cover of night, and eavesdropping to see what intelligence he can glean from conversations in the command bunker. He discovers that two massive railway howitzers (“Big Berthas”) are being brought to this position in advance of another French attack. When the French high command are informed of this, they fall to squabbling among themselves, ordering, cancelling, and re-ordering the attack. Indy finally “loses” the written orders in a staged motorcycle accident. He is sent back to the trenches, where he reunites with a recovered Remy.
October 1916 — Indy and Remy are granted a two-week furlough in Paris. Indy stays with a colleague of his father’s and, out of politeness, is forced to attend a high society dinner party rather than carouse in the bars and brothels with Remy. There he meets the exotic dancer Mata Hari, and he begins a passionate affair with the older woman. They are clandestinely followed wherever they go. Indy struggles with the fact that she is obviously seeing other men. He is finally confronted by British and French authorities, and warned to stay away from her. His two-week furlough is cancelled. (Mata Hari is later revealed to be a spy for Germany, and is executed the following year.)
November 1916 — Indy and Remy are promoted to lieutenants and transferred to the African front. They get lost on their way to their new posting near Nairobi, and stumble onto an Allied camp in the middle of the savannah. They are the 25th Royal Fusiliers, made up mostly of older veteran soldiers, and they have been tasked with destroying a massive German railroad gun that seems to be able to disappear at will. Reconnaissance by Indy and the Fusiliers uncovers a tunnel hidden behind an artificial cliff face. After a complicated hijacking of the train, they succeed in their mission. The Fusiliers then agree to escort Indy and Remy to back their unit. They are captured and get taken to the German camp commanded by the legendary General Von Letow-Vorbeck. Indy and the Fusiliers manage to turn the tables on Von Letow-Vorbevk, and take him hostage. Indy, Remy, and their hostage accidentally take flight in a hot air balloon. After traveling many miles, the balloon crashes in the wilderness. They escape an encounter with hostile tribesmen, and facing a rapidly approaching German unit, Indy and Remy let their prisoner go in order to make a quick escape. They are soon picked up by the Fusiliers.
Late November, 1916 — Back with his unit, Lt. “Defense” leads his mixed Belgian-African unit against an entrenched position under heavy fire. When Indy notices the enemy’s machine gun has jammed, he goes against Major Boucher’s withdrawal orders and continues the attack, which succeeds. Indy earns a promotion to captain, and the enmity of Major Boucher. Indy and the major are then ordered to lead a large expedition 2000 miles through the Congo region to retrieve a shipment of heavy machine guns that has foundered on the west coast of Africa, near Port Lopez. 
December 3, 1916 — The expedition across the Congo begins.
Mid-December, 1916 — The trans-Congo expedition encounters a village wiped out by smallpox, except for one small boy, who seems healthy. Major Boucher orders the boy be left behind to die, but the company defies his orders.
December 24, 1916 — Most of the company has perished from disease, and those that remain alive are very ill. Continue reading