All history seems to be the same as in our timeline up until that day in 1977, except the 1906 California Earthquake never happened, meaning that over the course of 71 years, pressure was building and building; so much so that when it ruptured in 1977 the whole world paid dearly for it!
The quake happened on April 19, 1977. The United States suffered greatly. The Empire State building collapsed, the Hoover dam broke, It was chaos.
January, 1978: Nine Months Later
Unable to deal with the pressure of the world situation, President Jimmy Carter steps down from the Presidency and his vice president, Walter Mondale, takes the seat. He believed that tougher leadership was needed in the time of crisis. The Congress was very much against everything he proposed and vice-versa. The feud between the executive and legislative branches became so heated that the Congress, for political reasons and nothing to do with the law, planned to impeach the President. Before they could do anything the President himself commanded a military unit to overrun Capitol Hill, and does so with ease. With little to no opposition he sets up a new government with him at the head of both the executive and legislative branches.
Meanwhile, Europe, a continent barely recovered from World War 2, has been all but destroyed. Buildings demolished to nothing but rubble; governments collapsing under the economic strain of trying to rebuild; the crime rate sky rockets and the death toll is unimagineable. Taking advantage of the situation, 5 months after a group calling themselves the Peacemen start overrunning small communities then the cities and then the country. Their leader is named Massimo DeCavalli. His tactics, however, are anything but peacful. He slaughters all who get in his way. He is absolutely ruthless when it comes to meeting out his goal. The armies that come against them are more often than not destroyed within hours. And when he sets up his regime in a conquered region he is merciless when seeking out "justice". His favorite form of execution: Crucifixion.
When Britain sees what's happening on the continent it summons up all the strength it can muster to break the Peacemen.
March, 1978: The First Shots of War are Heard
With DeCavalli's army in the heart of Germany, the United Kingdom makes its move in an attempt to defeat the Peacemen. By now however Massimo's forces had grown so large and well-trained (through threats of torture and execution) that at every encounter any sort of defense is futile. By the end of March Britain had suffered the loss of over 30,000 military personnel.
Summoning the US, Prime Minister Callaghan begs for support to defeat the conquerors. He points out that if Britain were to fall there would be no stopping the Peacemen from overrunning the eastern shore of the US. Mondale agrees to enforce a draft requiring all able people from 17-40 to enlist in a branch of the military. By July the US was ready to support Britain. But it was too late; the Peacemen had overrrun England.
July, 1978 - May 1979: One Disaster after Another
The United States listened into Britain. After hearing no transmissions, they decide to travel to Britain. A stealth fleet of Peacemen sneak into Washington D.C. and assassinate President Mondale. The US fleet turns back, and the Peacemen kill about 300 soldiers before they are killed themselves. At this point, the Peacemen have grown. They now have well over 10,000,000 men. The plans to conquer the planet come into fruition for DeCavalli...
June - August, 1979: Peace... But Not For Long
A period of peace follows as the United States reorganizes itself. Ex-President Jimmy Carter signs back on as the POTUS and begins a large plan. By July, large battle fleets crisscross the Western side of the Atlantic Ocean. If the battle fleets failed, the entire Western Seaboard was turned into a gigantic "Wall of the United States", a huge defensive barrier. Well over 200,000 men are posted there, and there are multiple anti-ship turrets. If the Peacemen got hold of British planes, the US had also invested in SAMs and Triple A cannons. As a last-ditch defensive, if they got hold of nuclear weapons, the US was constructing ground-based particle cannons to shoot them down, and converted old fighter planes into fast planes designed for nuclear missile interception. The Peacemen were also preparing. Sure enough, they had gotten hold of nuclear weapons, but scrapped them. They got hold of fighter planes too. A humongous fleet was built. The Peacemen, roughly 900,000 of them, traveled in newly-built Super Battleships and old WWII battleships. Their fleet numbered 8000 ships. The war was about to start... and this time, the United States stands no chance under the might of the Peacemen ...
August, 1979 - April, 1980: First Strike and the Battle for the Atlantic
The Americans, not wanting to wait for the invasion, launch a pre-emptive strike on the Peacemen fleet by bombarding them with every bomber they've got. The surprise attack is a successful bombardment resulting in the decimation of the Peacemen fleet. The losses of the Peacemen fleet die to the bombardment are massive(10 Super Battleships, 50 Aircraft Carriers, 550 WW-II era battleships and two of their three command ships) while on the other hand the Peacemen only managed to shoot down 2000 of the 120000 strong bomber squadron. This became known among the Americans as the Pearl Harbor of the war with the Peacemen and rose the morale of the US troops. Despite the losses the fleet reaches the American blockade and the siege of America begins.
The Peacemen launch wave attacks on the blockade but despite outnumbering the Americans 20:1 they were inexperienced in naval combat. The result being a stalemate going on for nine months with the Americans holding their ground. What originally was considered a sure victory for the Peacemen became a battle that didn't have a clear winner any time soon.
April 10, 1980: An Alliance is Formed
While the brave US sailors were defending the blockade President Carter contacts the Canadian and Mexican governments in the hope of making an alliance against the Peacemen. Although initially reluctant to join the two nations agree to aid USA following the news that the Peacemen finally broke through the blockade. With days away before they invade Washington D.C. the Mexican and Canadian forces needed to be ready or the fall of Washington might just mean the fall of Northern America.
April - May, 1980: The Battle for the East Coast
The Peacemen finally reached the American East Coast and the battles of Washington, New York, Philadelphia, and Boston began. And while the Peacemen were better trained than the American volunteers they suffered the consequences of being trapped at sea for nine months, again resulting in a stalemate with both sides having advantages against one another. The battles continued for two weeks until the Mexican fleet arrived to assist the American forces in Washington. With the capital secured, the remaining Peacemen troops quickly became outmatched by the American army and the Mexican fleet in the south and by the remnants of the American fleet and the Canadian reinforcements to the north. The liberation of New York on the 30th of May marked the end of the invasion, with the Peacemen retreating back to international waters. The news of the defeat at America soon reached the ears of DeCavalli who decided that if America wouldn't be conquered then maybe he could direct his attention towards ridding the world of the Soviet Union.