The Twilight of the Gods was the period that lasted from 400 ABY to 500 ABY, and saw the complete destruction of all Force using cults, and the extermination of Force users in a fashion unparalleled since the Jedi Purge. It resulted in the Echo Peace, a period of a century of complete peace.
The Restored Republic saw the resolution of the differences between the Empire and Galactic Alliance, with independence an almost completely abandoned concept. The Mandalorians stayed apart from the union, but the Jedi Order nonetheless kept them in check, should it come to it. The Republic represented a political solution to the past centuries of violence.
When tests on an flower named Echo resulted in a tool which could suppress midichlorians, the peace was shattered, as the Republic Senate seized upon the 'cure', and declaring Force sensitivity illegal. The Jedi Order split, caught between the possibility that the war over the Force could be ended permanently, and that the Jedi were to be also vaccinated.
The Gods of the Force were to die.
The Republic didn't wait for the Jedi to resolve their differences, and struck, hard, with everything at their disposal. The Hapans provided fleets to the Republic to suppress the Jedi, and a civil war began, as factions within the Republic sought to prevent a genocide. The Mon Calamari and Wookies, long friends of the Jedi, seceded, seeking to protect the enemy. Elements within the old Empire resurfaced, old malcontents pushing for a new Imperial state. Corellia, the old warhorse of independence, allied with Mon Calamari and Kashyyyk, dragging out the crisis.
The war dragged on for two decades, until the Republic managed to regain the initiative, focusing on the Mon Calamari as the cornerstone of the crisis. With them gone, the Corellians and Imperials would cease their games, as they would be too weak to stand alone, and unwilling to cooperate due to their histories of opposition. Pressing down on Mon Calamari, the Republic exchanged body-blows with the species while their fleets raided incessantly, and the Empire re-established itself only to be enmeshed in the crisis as the Imperial Knights sought to aid the Jedi.
The Republic finally defeated the Jedi and the Mon Calamari fleets at the Battle of Ossus, and the Empire and Corellians rushed back to the Republic, disarming their illegal gains before too harsh sanctions could be placed upon them. Corellia, for its part, was hit by a last-minute Republic attack, just before the Treaty of Ossus was signed, crippling their government, and Corellia was rudderless, re-integrated within weeks.
Jedi remnants fled to the depths of the Outer Rim, and the Republic pressed after them, growing more militant by the day. The Sith returned the moment after the Treaty of Ossus was signed, taking advantage of the immediate confusion to deal blows against Echo, an assault which was tailored against removing it from the galaxy, striking when all sides were at their weakest. The Republic inevitably rallied, as too did the Jedi, but the Restored Republic finally triumphed over both sides, a third decade rolling over before the Republic had restored control.
Skirmishes flared up for the next three decades, as Jedi and Sith were discovered from time to time by the Republic, but it's militarisation began to subside, as the need became less apparent. Stability was achieved, and the Senate went far enough to declare that the war was over in 460 ABY. It wasn't. In-fact, an even more terrible chapter in the Twilight was to be unveiled.
The Mandalorians had spent these six decades preparing for the day when the Jedi and Sith were no more. When they would be free to conquer the galaxy. Upon the declaration of peace, the Mandalorians acted, emerging from the Mandalorian Sector and driving a knife up the Hydian Way at Coruscant. Capturing as far coreward as Anaxes within three years, the Restored Republic rallied, gathering an unprecedented armada at Coruscant - only for the Mandalorians to drive at the key members of the Republic to divide their strength; Hapes, Kuat, Bastion and Csilla were struck at, a two year long feint which drained the Republic's strength in the Core. Billions died, with the Republic being caught entirely unaware. Expertly, the Mandalorians allowed themselves to lose their systems in the Core, allowing them to increase the strength of their assaults on the other key worlds. The Republic finally split their remaining combined fleets, knowing that holding Coruscant meant nothing if the rest of the Republic fell.
The moment the enemy scattered, the Mandalorians struck at Coruscant, which fell in a terrible bloodbath - but the Restored Republic had been anticipating that this was to happen, and moved the capital coreward, to Empress Teta. The Mandalorians, caught fighting the Siege of Coruscant, struggled to press into the Deep Core, and their feints became traps, the Mandalorians unable to abandon their offensives on any of their arenas without the Republic gathering strength from them. This impasse dragged on for three terrible years, Empress Teta fending off attack after attack from the enemy, and the Republic slowly recovering their threatened worlds one meter at a time.
Mon Calamari rededicated itself to the Republic in a tremendous diplomatic victory, and the Republic steadily began pushing the Mandalorians from the numerous fronts, until they advanced on Coruscant. But the Mandalorians had not yet surrendered. They abandoned their thrust coreward, leaving a trail of fortress worlds from the Rim to the Core, and used their forces to consolidate an enlargened Mandalore Sector in the Outer Rim, capturing a hundred sectors.
The Republic took more than a year to recapture the Hydian, and by that time the Mandalorians were entrenched in their new holdings. A strike at Mandalore achieved nothing, the Mandalorians ensuring the Republic attacked there first to draw them into the Outer Rim, the Mandalorians scattering to their new territories. The Mandalorians were ready, and it appeared as if the Republic might be ground down in the sieges, caught at the Siege of Toprawa. However, the Mandalorian leader was mysteriously assassinated, and the clans stopped cooperating - splitting into individual fiefdoms.
The tide had turned.
Apart from a few setbacks, the Restored Republic pushed at the Mandalorians until the group was officially disbanded, and their armour made as illegal as Force sensitivity. As the century rolled around, the Restored Republic was in complete control of the entirety of known space. There were no more threats on the horizon, and the Echo Peace followed, with the continued practice of innoculating children at birth with Echo taking root in the system. The Republic grew stronger than ever, keeping its capital on Empress Teta as a reminder as to why the continued militarisation must continue.
The Republic was not again threatened until the Archangels & Alchemists resurfaced...