diff --git a/MultiversalDiplomacy/Model/Unit.cs b/MultiversalDiplomacy/Model/Unit.cs
index 305d39f..a9cba9f 100644
--- a/MultiversalDiplomacy/Model/Unit.cs
+++ b/MultiversalDiplomacy/Model/Unit.cs
@@ -7,11 +7,6 @@ namespace MultiversalDiplomacy.Model;
 /// </summary>
 public class Unit
 {
-    /// <summary>
-    /// The previous iteration of a unit. This is null if the unit was just built.
-    /// </summary>
-    public string? Past { get; }
-
     /// <summary>
     /// The location on the map where the unit is.
     /// </summary>
@@ -38,9 +33,8 @@ public class Unit
     [JsonIgnore]
     public string Key => $"{Type.ToShort()} {Season.Timeline}-{Location}@{Season.Turn}";
 
-    public Unit(string? past, string location, Season season, string power, UnitType type)
+    public Unit(string location, Season season, string power, UnitType type)
     {
-        this.Past = past;
         this.Location = location;
         this.Season = season;
         this.Power = power;
@@ -55,13 +49,13 @@ public class Unit
     /// method after accepting a build order.
     /// </summary>
     public static Unit Build(string location, Season season, string power, UnitType type)
-        => new(past: null, location, season, power, type);
+        => new(location, season, power, type);
 
     /// <summary>
     /// Advance this unit's timeline to a new location and season.
     /// </summary>
     public Unit Next(string location, Season season)
-        => new(past: this.Key, location, season, this.Power, this.Type);
+        => new(location, season, this.Power, this.Type);
 
     public Location GetLocation(World world) => world.Map.GetLocation(Location);
 
diff --git a/MultiversalDiplomacyTests/MDATC_A.cs b/MultiversalDiplomacyTests/MDATC_A.cs
index b3de1c8..fcb41cb 100644
--- a/MultiversalDiplomacyTests/MDATC_A.cs
+++ b/MultiversalDiplomacyTests/MDATC_A.cs
@@ -47,12 +47,9 @@ public class TimeTravelTest
         Unit originalUnit = world.GetUnitAt("Mun", s0);
         Unit aMun0 = world.GetUnitAt("Mun", s1);
         Unit aTyr = world.GetUnitAt("Tyr", fork);
-        Assert.That(aTyr.Past, Is.EqualTo(mun1.Order.Unit.Key));
-        Assert.That(world.GetUnitByKey(aTyr.Past!).Past, Is.EqualTo(mun0.Order.Unit.Key));
 
         // Confirm that there is a unit in Mun b1 originating from Mun a0
         Unit aMun1 = world.GetUnitAt("Mun", fork);
-        Assert.That(aMun1.Past, Is.EqualTo(originalUnit.Key));
     }
 
     [Test]
@@ -94,10 +91,6 @@ public class TimeTravelTest
         World world = setup.UpdateWorld();
         Season fork = new("b1");
         Unit tyr1 = world.GetUnitAt("Tyr", fork);
-        Assert.That(
-            tyr1.Past,
-            Is.EqualTo(mun0.Order.Unit.Key),
-            "Expected A Mun a0 to advance to Tyr b1");
         Assert.That(
             world.RetreatingUnits.Count,
             Is.EqualTo(1),
diff --git a/MultiversalDiplomacyTests/MovementAdjudicatorTest.cs b/MultiversalDiplomacyTests/MovementAdjudicatorTest.cs
index 8b2be91..50e51e9 100644
--- a/MultiversalDiplomacyTests/MovementAdjudicatorTest.cs
+++ b/MultiversalDiplomacyTests/MovementAdjudicatorTest.cs
@@ -171,7 +171,7 @@ public class MovementAdjudicatorTest
 
         // Confirm the unit was created
         Assert.That(updated.Units.Count, Is.EqualTo(2));
-        Unit second = updated.Units.Single(u => u.Past != null);
+        Unit second = updated.Units.OrderBy(u => -u.Season.Turn).First();
         Assert.That(second.Location, Is.EqualTo(mun.Order.Unit.Location));
         Assert.That(second.Season.Timeline, Is.EqualTo(mun.Order.Unit.Season.Timeline));
 
@@ -208,7 +208,6 @@ public class MovementAdjudicatorTest
         Unit u2 = updated.GetUnitAt("Mun", s2);
         Assert.That(updated.Units.Count, Is.EqualTo(2));
         Assert.That(u2.Key, Is.Not.EqualTo(mun1.Order.Unit.Key));
-        Assert.That(u2.Past, Is.EqualTo(mun1.Order.Unit.Key));
         Assert.That(u2.Season, Is.EqualTo(s2));
 
         setup[("a", 1)]
@@ -228,7 +227,6 @@ public class MovementAdjudicatorTest
         updated = setup.UpdateWorld();
         Season s3 = new(s2.Timeline, s2.Turn + 1);
         Unit u3 = updated.GetUnitAt("Mun", s3);
-        Assert.That(u3.Past, Is.EqualTo(mun2.Order.Unit.Key));
     }
 
     [Test]
@@ -258,7 +256,6 @@ public class MovementAdjudicatorTest
         Unit u2 = updated.GetUnitAt("Tyr", s2);
         Assert.That(updated.Units.Count, Is.EqualTo(2));
         Assert.That(u2.Key, Is.Not.EqualTo(mun1.Order.Unit.Key));
-        Assert.That(u2.Past, Is.EqualTo(mun1.Order.Unit.Key));
         Assert.That(u2.Season, Is.EqualTo(s2));
 
         setup[("a", 1)]
@@ -278,6 +275,5 @@ public class MovementAdjudicatorTest
         updated = setup.UpdateWorld();
         Season s3 = new(s2.Timeline, s2.Turn + 1);
         Unit u3 = updated.GetUnitAt("Mun", s3);
-        Assert.That(u3.Past, Is.EqualTo(u2.Key));
     }
 }
diff --git a/MultiversalDiplomacyTests/SerializationTest.cs b/MultiversalDiplomacyTests/SerializationTest.cs
index 91ca56e..34a37d2 100644
--- a/MultiversalDiplomacyTests/SerializationTest.cs
+++ b/MultiversalDiplomacyTests/SerializationTest.cs
@@ -133,10 +133,6 @@ public class SerializationTest
         World world = setup.UpdateWorld();
         Season fork = new("b1");
         Unit tyr1 = world.GetUnitAt("Tyr", fork);
-        Assert.That(
-            tyr1.Past,
-            Is.EqualTo(mun0.Order.Unit.Key),
-            "Expected A Mun a0 to advance to Tyr b1");
         Assert.That(
             world.RetreatingUnits.Count,
             Is.EqualTo(1),
diff --git a/MultiversalDiplomacyTests/UnitTests.cs b/MultiversalDiplomacyTests/UnitTests.cs
index f10dac6..5293be4 100644
--- a/MultiversalDiplomacyTests/UnitTests.cs
+++ b/MultiversalDiplomacyTests/UnitTests.cs
@@ -22,10 +22,6 @@ public class UnitTests
         _ = world.WithNewSeason(a1, out Season a2);
         Unit u3 = u2.Next(Tyr.Key, a2);
 
-        Assert.That(u3.Past, Is.EqualTo(u2.Key), "Missing unit past");
-        Assert.That(u2.Past, Is.EqualTo(u1.Key), "Missing unit past");
-        Assert.That(u1.Past, Is.Null, "Unexpected unit past");
-
         Assert.That(u1.Season, Is.EqualTo(a0), "Unexpected unit season");
         Assert.That(u2.Season, Is.EqualTo(a1), "Unexpected unit season");
         Assert.That(u3.Season, Is.EqualTo(a2), "Unexpected unit season");