In Code Geass, the invasion of Japan in its full extent is told through bits and pieces over a lot of episodes, which makes it hard to piece together what was the actual timeline from the decision being made in Britannia to the conclusion of the invasion. What were the steps (at least the major ones) between those two points?
With lots of quoting from World history code geass
Note: the world history of code geass mainly resolves around big changes such as wars and arrival of mysterious things such as C.C. a.t.b stands for "Ascension Throne Britannia"
55 B.C. or 1 a.t.b. Julius Caesar attempts to invade Britain, but is met with strong resistance from the local tribes, who elect a super-leader: the Celtic King Eowyn, who summarily becomes the first member of the Britannian Imperial Family. The Julian Calendar is devised ten years later and the Anno Domini Calendar 470 years later. Either the two might be used by other countries aside from Britannia and its colonies.
Unknown years - Middle Ages Sakuradite (known as the "Philosopher's Stone" at the time) is discovered near Stonehenge. The scarcity of sakuradite impedes the research to turn it into a viable energy source. However, in his travels, Marco Polo journeys further into the East, reaching Japan and discovering the nation's large deposit of sakuradite.
17th Century a.t.b. Elizabeth I, who remained single throughout her life, bears a son, Henry IX. The potential fathers — Sir Robert Dudley, 1st Earl of Leicester; Sir Robert Devereux, 2nd Earl of Essex; and Sir Carl, Duke of Britannia — gain influence and power with this knowledge. Henry IX ascends to the throne after the death of his mother in 1603 A.D. or 1657 a.t.b., starting the Golden Age of the Tudor Dynasty.
1820s a.t.b. / 1760s - 70s A.D. The American Revolution (also known as Washington's Rebellion) occurs. The Duke of Britannia bribes Benjamin Franklin with promises of titles and territories in the colonies, who was charged with appealing to Louis XVI for assistance in the American colonies' war for independence. Thereafter, Benjamin Franklin is given the title of Earl. As a result, the Continental Army suffers a decisive defeat during the Siege of Yorktown with the death of George Washington, marking a severe blow to the American movement for independence.
Middle of the 19th Century a.t.b. The western world enters the Age of Revolution, with numerous national revolutions taking place, except in the British Isles, under the rule of King Henry X, who continues to hold absolute monarchy. This leads to the formation of the European Union. After French revolutionary Napoleon Bonaparte emerges, he wins the Battle of Trafalgar, invades Great Britain, and occupies London. The British Isles are conquered and become a part of the E.U. In 1807 A.D./ 1861 a.t.b., Queen Elizabeth III retreats to Edinburgh, where a revolutionary militia arrests her and forces her to abdicate, ending the monarchy. This event becomes known as The Humiliation of Edinburgh. However, Sir Ricardo von Britannia, Duke of Britannia, and his friend and subordinate, Sir Richard Hector, Knight of One, bring Elizabeth III and her followers to the New World and establish a capital on the east coast of North America. The British Isles is now under control of the EU and a new government established.
1867 a.t.b. / 1812 A.D. Elizabeth III nominates her lover, Sir Ricardo von Britannia, as her successor upon her death. She ends her reign for being "the Queen who loved throughout her stormy life". The Britannian calendar, Ascension Throne Britannia (a.t.b.), is established, with the origin year set to the ascendancy of the first Celtic king, though the calendar's months and days are adopted from the Gregorian Calendar.
1874 a.t.b. / 1819 A.D. Napoleon Bonaparte dies on his way back to France, after his defeat at the Battle of Waterloo; although it was never proven, it is rumored that assassins poisoned his food in accordance to Elizabeth III's will. Her last words included the famous line, "I do not forget slights to my honour."
Unknown years - Late 19th to Early 20th Century a.t.b. A war featuring tanks and trenches is fought as seen in C.C.'s flashbacks in Episode 25 of R1, which was probably fought in the EU. Also in the flashback, C.C. was shot by possibly a German soldier (considering the silhouette).
1944 a.t.b. / 1889 A.D. Japan loses a major war (described in 2010 a.t.b as having taken place 65 years earlier) embracing democracy as a result (Mamoru Iwasa, Code Geass Stage-0-Entrance Light Novel, p.120-121).
1984 a.t.b. / 1929 A.D. 1st Prince Odysseus eu Britannia is born.
1986 a.t.b. / 1931 A.D. 1st Princess Guinevere su Britannia is born.
1990 a.t.b. / 1935 A.D. 2nd Prince Schneizel el Britannia is born.
1991 a.t.b. / 1936 A.D. 2nd Princess Cornelia li Britannia is born.
1992 a.t.b. / 1937 A.D. 3rd Prince Clovis la Britannia is born.
1998 a.t.b. / 1943 A.D. The 97th Emperor of Britannia is overthrown, and Charles zi Britannia ascends to the Britannian throne. Charles also marries Marianne vi Britannia. With V.V. they plan to build a weapon to destroy the Gods.
2000 a.t.b. / 1945 A.D. 11th Prince Lelouch vi Britannia is born Suzaku Kururugi, son of Japanese Prime Minister Genbu Kururugi and future Knight of Seven, Knight of Zero, and second Zero, is born.
2001 a.t.b. / 1946 A.D. 3rd Princess Euphemia li Britannia is born.
2003 a.t.b. / 1948 A.D. 4th Princess Nunnally vi Britannia is born. 5th Princess Carine ne Britannia is born.
2009 a.t.b. / 1954 A.D. Marianne vi Britannia is killed by V.V. Her children, Lelouch vi Britannia and Nunnally vi Britannia, are sent to Japan as political hostages.
2010 a.t.b / 1955 A.D. After the Indochinese Peninsula was conquered by Britannia and named Area 10, Japan, which was originally neutral, decided to align its policy with Chinese Federation and EU's politics and apply economic pressure upon Britannia -- an event referred to as the Oriental Incident. Both the Chinese Federation, the EU and their allies blockade the ports of Britannia in an effort to come to negotiations.
August 10th, 2010 a.t.b / 1955 A.D. The Second Pacific War erupts; the one-month war results in the conquering of Japan by Britannia. The end of the war signifies Japan as a formal colony of Britannia, renamed Area 11 and its citizens "Elevens".
2017 a.t.b. / 1962 A.D. Viceroy and Third Prince Clovis la Britannia is killed by Zero. Second Princess Cornelia li Britannia is appointed Viceroy of Area 11, naming Third Princess Euphemia li Britannia as sub-viceroy. Cornelia immediately begins her campaign to bring Zero to justice. Zero forms the Order of the Black Knights. Its membership expands with each victory. All attempts to destroy the Black Knights end in failure. Using her power, Euphemia li Britannia tries to create a Special Administrative Zone of Japan. It is "revealed" to be a plot to massacre the Elevens, and she is killed by Zero. The Black Rebellion breaks out. The Black Knights ignite riots across the country as they continue to push towards the Area 11 Viceroy Palace. The rebellion is eventually pushed back and crushed, with most of the Black Knights killed or captured. Area 11 is demoted to a correctional sub-area. Second Princess Cornelia li Britannia is gone missing during the Black Rebellion. Her position as Viceroy of Area 11 later is taken by Calares.
2018 a.t.b. / 1963 A.D. The remaining members of the Black Knights incite riots in Babel Tower which kill Calares. Within the Chinese Federation Consulate of Area 11, Zero reappears and redeclares the United States of Japan. The former Knight of Princess Cornelia, Gilbert G. P. Guilford, declares himself as new Viceroy. He then announces to execute Black Knights' members, although it's failed thanks to Zero's crafty strategy. Princess Nunnally vi Britannia becomes Viceroy of Area 11, and reestablishes the Special Administration Zone of Japan. Zero gives his support on the condition that he be exiled. Zero, along with over a million of his supporters dressed like him, are exiled from Area 11 and seek asylum on Penglai Island located within the political borders of the Chinese Federation. Li Xingke and Zero disrupt the arranged political wedding between First Prince Odysseus eu Britannia and Empress Tianzi of the Chinese Federation. The High Eunuchs are summarily executed for lèse majesté against the Empress. An alliance between the Black Knights and the Chinese Federation is forged. After the fragmentation of the Chinese Federation, Second Prince Schneizel el Britannia prepares to annex many of its territories through a combination of diplomatic means, which results in failure. Ratification of United Federation of Nations is completed, with the Order of the Black Knights to serve as its chief military branch. Its first resolution is to take action against the occupying Britannian forces in Japan, which leads to a declaration of war between the U.F.N. and the Holy Britannian Empire. The U.F.N. initiates the Second Battle of Tokyo to reclaim Area 11. The first use of F.L.E.I.J.A. by the Britannians results in massive damages to the concession area of the Tokyo Settlement. Zero is reported to be killed during the Second Battle of Tokyo. However, there are rumors that this is falsified information for reasons unknown. In light of the alleged death of Zero, a truce is negotiated between U.F.N. and Britannia, with a peace treaty being drafted. The treaty results in the neutrality of Japan. A coup d'etat is enacted by Second Prince Schneizel and the Knight of Seven, Suzaku Kururugi. Meanwhile, Lelouch vi Britannia instigates his own rebellion, forcing his followers to his will with the power of Geass. 98th Emperor, Charles zi Britannia is killed by Lelouch, the former 11th Prince. One month after the Second Battle of Tokyo, Lelouch vi Britannia coronates himself as the 99th Emperor of the Holy Britannian Empire and appoints the title of "Knight of Zero" to Suzaku Kururugi (heretofore the Knight of Seven). With his ascension come many changes in Britannian policies, including the destruction of the Imperial Mausoleum and the abolishment of privileges to nobles. This leads to many attempts of rebellion (one of which is led by the Knight of One, Bismarck Waldstein), each of them futile against the "Emperor of Justice". Britannia attempts to join the U.F.N., and negotiation takes place in Japan, now a neutral zone. However, the leaders of the U.F.N. and the Black Knights attempt to convince Emperor Lelouch to lower Britannia's voting rights as a balance for the U.F.N.. The attempt fails, and in response Britannia launches an invasion against Japan, capturing the leadership of the U.F.N.. F.L.E.I.J.A. bomb is dropped on the Britannian capital of Pendragon from the Aerial Fortress Damocles, created by the Toromo institute, by Former 2nd Prince Schneizel resulting in the complete decimation of the capital. The battle of Mount Fuji begins. Both sides suffer heavy losses, but eventually Emperor Lelouch is able to take control of Damocles and ends the battle, as well as the war between Britannia and the U.F.N., with a display of power from the Damocles. Two months after the battle, using U.F.N. as political tool for forcing E.U. to raficate, Emperor Lelouch vi Britannia declares himself world leader. During a supervision of executions of the Black Knights and U.F.N. leaders, Zero reappears and assassinates Lelouch to complete the Zero Requiem. Nunnally vi Britannia succeeds her elder brother as the 100th Empress of Britannia and begins to work in cooperation with the U.F.N. to achieve peace.