diff --git a/MultiversalDiplomacy/Model/Unit.cs b/MultiversalDiplomacy/Model/Unit.cs
index e303629..e1bda17 100644
--- a/MultiversalDiplomacy/Model/Unit.cs
+++ b/MultiversalDiplomacy/Model/Unit.cs
@@ -8,7 +8,7 @@ public class Unit
     /// <summary>
     /// The previous iteration of a unit. This is null if the unit was just built.
     /// </summary>
-    public Unit? Past { get; }
+    public string? Past { get; }
 
     /// <summary>
     /// The location on the map where the unit is.
@@ -30,7 +30,12 @@ public class Unit
     /// </summary>
     public UnitType Type { get; }
 
-    private Unit(Unit? past, string location, Season season, Power power, UnitType type)
+    /// <summary>
+    /// A unique designation for this unit.
+    /// </summary>
+    public string Designation => $"{Type.ToShort()} {Season.Timeline}-{Location}@{Season.Turn}";
+
+    private Unit(string? past, string location, Season season, Power power, UnitType type)
     {
         this.Past = past;
         this.Location = location;
@@ -53,5 +58,5 @@ public class Unit
     /// Advance this unit's timeline to a new location and season.
     /// </summary>
     public Unit Next(string location, Season season)
-        => new(past: this, location, season, this.Power, this.Type);
+        => new(past: this.Designation, location, season, this.Power, this.Type);
 }
diff --git a/MultiversalDiplomacy/Model/World.cs b/MultiversalDiplomacy/Model/World.cs
index dc5b802..530086b 100644
--- a/MultiversalDiplomacy/Model/World.cs
+++ b/MultiversalDiplomacy/Model/World.cs
@@ -332,4 +332,8 @@ public class World
             ?? throw new KeyNotFoundException($"Unit at {province} at {season} not found");
         return foundUnit;
     }
+
+    public Unit GetUnitByDesignation(string designation)
+        => Units.SingleOrDefault(u => u!.Designation == designation, null)
+        ?? throw new KeyNotFoundException($"Unit {designation} not found");
 }
diff --git a/MultiversalDiplomacyTests/MDATC_A.cs b/MultiversalDiplomacyTests/MDATC_A.cs
index 59887e3..3d09311 100644
--- a/MultiversalDiplomacyTests/MDATC_A.cs
+++ b/MultiversalDiplomacyTests/MDATC_A.cs
@@ -46,12 +46,12 @@ 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));
-        Assert.That(aTyr.Past?.Past, Is.EqualTo(mun0.Order.Unit));
+        Assert.That(aTyr.Past, Is.EqualTo(mun1.Order.Unit.Designation));
+        Assert.That(world.GetUnitByDesignation(aTyr.Past!).Past, Is.EqualTo(mun0.Order.Unit.Designation));
 
         // 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));
+        Assert.That(aMun1.Past, Is.EqualTo(originalUnit.Designation));
     }
 
     [Test]
@@ -95,7 +95,7 @@ public class TimeTravelTest
         Unit tyr1 = world.GetUnitAt("Tyr", fork);
         Assert.That(
             tyr1.Past,
-            Is.EqualTo(mun0.Order.Unit),
+            Is.EqualTo(mun0.Order.Unit.Designation),
             "Expected A Mun a0 to advance to Tyr b1");
         Assert.That(
             world.RetreatingUnits.Count,
diff --git a/MultiversalDiplomacyTests/MovementAdjudicatorTest.cs b/MultiversalDiplomacyTests/MovementAdjudicatorTest.cs
index 100e624..0874058 100644
--- a/MultiversalDiplomacyTests/MovementAdjudicatorTest.cs
+++ b/MultiversalDiplomacyTests/MovementAdjudicatorTest.cs
@@ -209,7 +209,7 @@ public class MovementAdjudicatorTest
         Unit u2 = updated.GetUnitAt("Mun", s2);
         Assert.That(updated.Units.Count, Is.EqualTo(2));
         Assert.That(u2, Is.Not.EqualTo(mun1.Order.Unit));
-        Assert.That(u2.Past, Is.EqualTo(mun1.Order.Unit));
+        Assert.That(u2.Past, Is.EqualTo(mun1.Order.Unit.Designation));
         Assert.That(u2.Season, Is.EqualTo(s2));
 
         setup[("a", 1)]
@@ -229,7 +229,7 @@ public class MovementAdjudicatorTest
         updated = setup.UpdateWorld();
         Season s3 = updated.GetSeason(s2.Timeline, s2.Turn + 1);
         Unit u3 = updated.GetUnitAt("Mun", s3);
-        Assert.That(u3.Past, Is.EqualTo(mun2.Order.Unit));
+        Assert.That(u3.Past, Is.EqualTo(mun2.Order.Unit.Designation));
     }
 
     [Test]
@@ -259,7 +259,7 @@ public class MovementAdjudicatorTest
         Unit u2 = updated.GetUnitAt("Tyr", s2);
         Assert.That(updated.Units.Count, Is.EqualTo(2));
         Assert.That(u2, Is.Not.EqualTo(mun1.Order.Unit));
-        Assert.That(u2.Past, Is.EqualTo(mun1.Order.Unit));
+        Assert.That(u2.Past, Is.EqualTo(mun1.Order.Unit.Designation));
         Assert.That(u2.Season, Is.EqualTo(s2));
 
         setup[("a", 1)]
@@ -279,6 +279,6 @@ public class MovementAdjudicatorTest
         updated = setup.UpdateWorld();
         Season s3 = updated.GetSeason(s2.Timeline, s2.Turn + 1);
         Unit u3 = updated.GetUnitAt("Mun", s3);
-        Assert.That(u3.Past, Is.EqualTo(u2));
+        Assert.That(u3.Past, Is.EqualTo(u2.Designation));
     }
 }
diff --git a/MultiversalDiplomacyTests/SerializationTest.cs b/MultiversalDiplomacyTests/SerializationTest.cs
index 44be89c..73c5fdf 100644
--- a/MultiversalDiplomacyTests/SerializationTest.cs
+++ b/MultiversalDiplomacyTests/SerializationTest.cs
@@ -94,7 +94,7 @@ public class SerializationTest
         Unit tyr1 = world.GetUnitAt("Tyr", fork);
         Assert.That(
             tyr1.Past,
-            Is.EqualTo(mun0.Order.Unit),
+            Is.EqualTo(mun0.Order.Unit.Designation),
             "Expected A Mun a0 to advance to Tyr b1");
         Assert.That(
             world.RetreatingUnits.Count,
diff --git a/MultiversalDiplomacyTests/UnitTests.cs b/MultiversalDiplomacyTests/UnitTests.cs
index 97611d0..3090fba 100644
--- a/MultiversalDiplomacyTests/UnitTests.cs
+++ b/MultiversalDiplomacyTests/UnitTests.cs
@@ -23,8 +23,8 @@ public class UnitTests
         _ = world.ContinueOrFork(a1, out Season a2);
         Unit u3 = u2.Next(Tyr.Designation, a2);
 
-        Assert.That(u3.Past, Is.EqualTo(u2), "Missing unit past");
-        Assert.That(u2.Past, Is.EqualTo(u1), "Missing unit past");
+        Assert.That(u3.Past, Is.EqualTo(u2.Designation), "Missing unit past");
+        Assert.That(u2.Past, Is.EqualTo(u1.Designation), "Missing unit past");
         Assert.That(u1.Past, Is.Null, "Unexpected unit past");
 
         Assert.That(u1.Season, Is.EqualTo(a0), "Unexpected unit season");